In droplet-based microfluidics, adding reagents into drops is one of the most important functions. In this paper, we develop a new picoinjector to add controlled volumes of reagent into droplets at kilohertz rates. In the new picoinjector, the adding process is triggered by the coining droplet itself, and the dosing volume can be precisely controlled. Meanwhile, the trigger voltage is lowered to several volts. These features make the system more practical and reliable. We expect the new picoinjector will find important applications of droplet-based microfluidics in automated biological assay, directed evolution, enzyme assay, and so on.