In this work, we demonstrated that p-type and n-type conduction could be induced in unintentionally doped pristine monolayer (ML) WS2 by forming a hybrid WS2/InGaN quantum dots (QDs) heterostructure, in which the ML-WS2 is partially covered by few-layer graphene. Under illumination, the photo-generated holes or electrons in the QDs were injected vertically into the ML-WS2 and then transported laterally therein. The polarity of the WS2 channel can be controlled by the bias applied to the graphene electrode. This work provides a potential approach to develop ambipolar devices of ML transition metal dichalcogenides through photocarrier doping.