When manual testing food and chemical samples, there are problems such as large pretreatment errors and low efficiency. In order to solve those problems, a adding liquid instrument control system for sample pretreatment was designed. The system adopts PLC as the main controller, and the software and hardware of the system were designed according to the control demand analysis of the pretreatment instruments. Combined the characteristics of the S-type acceleration and deceleration algorithm and non-contact liquid addition method. By controlling the acceleration of the deceleration phase during the liquid addition process for the droplet hanging on the tip falling off better, a segmented motor control model suitable for adding liquid was proposed. Finally, the fitting analysis and error compensation were performed on the dosing volume. The experimental results showed that the adding liquid error is within ±0.5%. less than the specified ±1%, whitch satisfies the adding liquid precision requirements of the pretreatment instrument, and has 96 sample positions, which is suitable for pretreatment of large batches of samples.