With respect to a liquid-fueled molten salt reactor (MSR), the temperature coefficient of reactivity mainly includes the moderator temperature coefficient (MTC) and the fuel salt temperature coefficient (FSTC). The FSTC is typically divided into the Doppler coefficient and the density coefficient. In order to compensate for the potentially positive MTC, the FSTC should be sufficiently negative, and this is mostly optimized in terms of the geometry aspect in pioneering studies. However, the properties of fuel salt also directly influence the FSTC. Thus, the effects of different fuel salt compositions including the 235\documentclass[12pt]{minimal}
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\begin{document}$$^{235}$$\end{document}U enrichment, heavy metal proportion in salt phase (HM proportion), and the 7\documentclass[12pt]{minimal}
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\begin{document}$$^{7}$$\end{document}Li enrichment on FSTC are investigated from the viewpoint of the essential six-factor formula. The analysis is based on an under-moderated MSR. With respect to the Doppler coefficient, the temperature coefficient of the fast fission factors (αT(ε)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(\varepsilon )$$\end{document}) is positive and those of the resonance escape probability (αT(p)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(f )$$\end{document}), and total non-leakage probability (αT(Λ)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(\varLambda )$$\end{document}) are negative. With respect to the density coefficient, αT(p)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(\eta )$$\end{document} are positive, while the others are negative. The results indicate that the effects of the 235\documentclass[12pt]{minimal}
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\begin{document}$$^{235}$$\end{document}U enrichment and HM on FSTC are mainly reflected in αT(ε)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(p )$$\end{document}, which are the dominant factors when the neutron spectrum is relatively hard. Furthermore, the 7\documentclass[12pt]{minimal}
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\begin{document}$$^{7}$$\end{document}Li enrichment influences FSTC by αT(f)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(\varLambda )$$\end{document}, which are the key factors in a relative soft spectrum. In order to obtain a more negative FSTC for an under-moderated MSR, the possible positive density coefficient, especially its αT(p)\documentclass[12pt]{minimal}
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\begin{document}$$\alpha _{\text {T}}(p )$$\end{document}, should be suppressed. Thus, a lower 235\documentclass[12pt]{minimal}
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\begin{document}$$^{235}$$\end{document}U enrichment (albeit higher than a certain value, 5 wt% in this article) along with a lower HM proportion and/or a higher 7\documentclass[12pt]{minimal}
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\begin{document}$$^{7}$$\end{document}Li enrichment are recommended. The analyses provide an approach to achieve a more suitable fuel salt composition with a sufficiently negative FSTC.
