Alloying/doping molybdenum disulfide (MoS₂) is a new way of exploring the potential applications of two-dimensional materials in microelectronic devices. Chemical vapor deposition is applied, for which sodium chloride is used to assist growth. Moreover, the mass ratio of sulfur powder to selenium powder is adjusted, and six kinds of monolayer MoS_2(1-x)Se_2x alloys with different compositions are thereby obtained on SiO₂/Si substrates. The photoluminescence peak position varied between 678 nm (~1.83 eV) and 813 nm (~1.53 eV). The transverse dimension of the continuously grown large-area monolayer MoS_2(1-x)Se_2x (x=0.25) alloys can reach 200 μm. For the investigation of the photoelectric properties of MoS_2(1-x)Se_{2xalloys, a large-area monolayer MoS2(1-x)Se2x (x=0.25) alloy is used to prepare field-effect transistors. The photoelectric test results show that the response of the monolayer MoS2(1-x)Se2x(x=0.25) field-effect transistor irradiated by the 520 nm laser reaches 940 mA·W^-1, with a detection rate of 5.32×10¹⁰ cm·Hz^1/2·W^-1 and a fast response time of 8 ms.}