Two-dimensional (2D) Janus transition metal dichalcogenides (JTMDs) exhibit suitable band gaps and strong visible-light absorption, which are extensively applied to the field of optoelectronic devices. Here, we investigate the electronic properties of 2D JTMDs MSeS (M=Hf, Zr) and SnS2 van der Waals heterojunction through density functional theory. The calculated electronic properties reveal that ZrSeS/SnS2 heterojunction has a type-I band alignment, while HfSeS/SnS2 heterojunction has a type-II band alignment. We build the diodes based on the MSeS (M=Hf, Zr)/SnS2 heterojunctions and study the electronic transport. The currents of the devices exhibit the asymmetry and the negative turn-on voltages suggest that constructed devices are backward diodes. Moreover, it is found that the gate voltage can modulate the rectifying ratio, and the rectifying performance of ZrSeS/SnS2 is better than that of HfSeS/SnS2.