Alloy engineering is of great importance in expanding the two-dimensional (2D) material family and modulating the bandgap of atomically thin 2D transition metal dichalcogenides (TMDs). Here, we report the synthesis of large area and high quality 2H Mo1-xRexS2 (x<=0.02) monolayer crystals by NaCl-assisted and space-confined chemical vapor deposition strategy. The combined effects of both common salt and confined space help promote the growth of Mo1-xRexS2 alloy monolayer crystals. The as-grown Mo1-xRexS2 alloy monolayers are uniformly distributed on the whole growth substrates with a domain size of up to 90 μm. Optical images, X-ray spectroscopy, Raman and photoluminescence spectroscopy measurements and corresponding mapping figures reflect the high quality and uniformity of the monolayers. The Mo1-xRexS2 alloy based phototransistor device exhibit good photoresponse to visible light with a response time of less than 0.2 s. Such alloy engineering of atomically thin 2D TMDs is useful for their future applications in optoelectronics.