Controllable phase modulation and electronic structure are essential factors in the study of two-dimensional transition metal dichalcogenides due to their impact on intriguing physical properties and versatile optoelectronic applications. Here, we report the phase-controlled growth of ternary monolayer MoSe2xTe2(1−x) (0 ≤ x ≤ 1) alloys induced through in situ doping and composition tuning via molecular beam epitaxy. Our approach leverages the substitution of selenium for tellurium to lower the energy barrier of the semi-conducting 2H and semi-metallic 1T′ phase transition. The alloys’ lattice constants, Mo-3d binding energy and electronic bandgap were demonstrated to be tunable by varying the selenium composition (x), respectively. First-principles calculations agree well with our experimental results, revealing that the valence band bowing effect of the monolayer alloys is attributed to the difference in coupling between anions and cations. This work provides a new pathway for phase modulation growth and controllable electronic structure of ternary monolayer transition metal dichalcogenide alloys, which is of great significance for ohmic contact and band engineering in developing transistor device applications using two-dimensional semiconductors.