The transport mechanism of photogenerated carriers in the composite films based on poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4,-phenylene-vinylene] (MEH-PPV) doped with CuS nanoparticles is investigated by the steady-state photo-current spectra and dynamic double-pulse photocurrent response. An obvious charging effect is found at lower concentration of CuS nanoparticles, while at high concentration, the same feature is absent. The charging effect is attributed to charge accumulation at the interface between MEH-PPV and CuS nanoparticles, which directly destroys the photocurrent responses of composite devices. It is concluded that doping CuS into MEH-PPV at low concentration causes trap states, and filling and emptying traps leads to charging and discharging effect respectively. At high concentration of CuS nanoparticles, the aggregation scale becomes larger and the double polar transport of charge carriers is improved, which inhibits the charge accumulation at the interface of MEH-PPV/CuS.