Because the dc side voltage value filter has a great influence on the compensation effect of active power filter,the article mainly studies the value selection and the control of dc side voltage for active power filter. It designs the experiment system of active power filter, and on this basis, the theoretical analysis of the mathematical relationship between the dc side voltage and offset current is done;Through experiments under different conditions the relationship between the compensation characteristic and dc side voltage is analyzed;According to the startup characteristic of the active power filter, It designs the corresponding PI algorithm and eliminates the unfavorable factor such as large overshoot of capacitance voltage at the startup phase. Finally it realizes the reasonable choice and effective control of dc side voltage.