Trions, quasi-particles consisting of two electrons combined with one hole or of two holes with one electron, have recently been observed in transition metal dichalcogenides (TMDCs) and drawn increasing attention due to potential applications of these materials in light-emitting diodes, valleytronic devices as well as for being a testbed for understanding many-body phenomena. Therefore, it is important to enhance the trion emission and its stability. In this study, a MoSe₂/FePS₃ van der Waals heterostructure (vdWH) with type-I band alignment is constructed, which allows for carriers injection from FePS₃ to MoSe₂. At low temperatures, the neutral exciton (X⁰) emission in this vdWH is almost completely suppressed. The I_Trion/I_x0 intensity ratio increases from 0.44 in a single MoSe₂ monolayer to 20 in this heterostructure with the trion charging state changing from negative in the monolayer to positive in the heterostructure. The optical pumping with circularly polarized light shows a 14% polarization for the trion emission in MoSe₂/FePS₃. Moreover, forming such type-I vdWH also gives rise to a 20-fold enhancement of the room temperature photoluminescence from monolayer MoSe₂. These results demonstrate a novel approach to convert excitons to trions in monolayer 2D TMDCs via interlayer doping effect using type-I band alignment in vdWH.