This paper is concerned with cooperative control for distributed grid-connected PV system. A two-layer control strategy is adopted, while the upper layer is a cooperative control to organize all the PV units to work cooperatively with given power ratio and the lower layer is a tracking control for a single PV unit. The information communication between PV units is realized through the ring communication structure applied in the upper layer. By using a mixed sensitivity approach, the controller for each single PV unit has strong robustness. The effectiveness of the proposed method is demonstrated by simulation in Matlab.