Van der Waals (vdW) heterojunctions based on two-dimensional materials have attracted great attention in emerging photoelectronics. However, the low-efficiency growth of single crystals significantly limits the practical applications of vdW heterojunctions. Here, we report a macro SnSe/SnSe2 heterojunction assembled by conformally transferring in-plane p-type SnSe on n-type SnSe2 synthesized by chemical vapor deposition. With well-matched band alignments, the SnSe/SnSe2 vdW photodetector exhibits dramatically enhanced performance with a responsivity of 17.5 mA W-1 and a response time of 17 ms, comparing with the sole SnSe or SnSe2 based photodetector.