The heterojunction device based on two-dimensional materials possesses unique photoelectric properties due to its nanoscale thickness and van der Waals (vdWs) contact surface. In this paper, a gate-voltage-tunable MoS₂/MoTe₂ vertical vdWs heterojunction device is constructed. The Kelvin probe force microscopy (KPFM) technology is combined with the electric transport measurement technology, thereby revealing the charge transport behavior of the MoS₂/MoTe₂ heterojunction under dark condition and laser-irradition condition, including the bipolarity characteristics of the transition from n-n⁺ junction to p-n junction. In this paper, the charge transport mechanism of heterojunction is explained comprehensively and systematically, including the charge transmission process of n-n⁺ junction and p-n junction under positive and negative bias conditions, the transformation of nodule behavior with gate voltage, the influence of barriers on charge transmission, the different rectification characteristics between n-n⁺ junction and p-n junction, the major role of source and leakage bias voltage in band tunneling, and the influence of photogenerated carriers on electrical transmission. The method in this work can be generalized to other two-dimensional heterojunction systems and also provide an important reference for improving the performance of two-dimensional semiconductor devices and their applications in the future.