Solid solutions forming and doping is an effective approach to optimize the transport properties of thermoelectric materials. In this study, a series of single-phase Mo_1-xW_xSeTe (0≤x≤0.5) solid solutions and their Nb-doped products were successfully synthesized with solid-state reaction followed by rapid sintering utilizing a Plasma Assisted Sintering apparatus. Thermoelectric transport studies showed that the carrier concentration, carrier mobility, electrical conductivity and power factor of the Nb_2yMo_0.5-yW_0.5-ySeTe solid solutions were significantly increased by W substitution and Nb doping, while their lattice thermal conductivity was reduced, leading to remarkably enhanced dimensionless figure of merit ZT. The simultaneous increment of carrier density and carrier mobility with the increasing Nb content is due to the transition from discrete energy levels to continuous impurity band through Nb doping. The study of anisotropy indicated that, Nb_2yMo_0.5-yW_0.5-ySeTe solid solutions owned a higher ZT value along the //P direction as a result of the lower lattice thermal conductivity. The Nb_0.03Mo_0.485W_0.485SeTe compound presented the highest ZT values among all samples, which were 0.31 and 0.36 (@823 K) along the ⊥p and //P directions, respectively, representing one of the best results based on MoSe₂-based materials. The enhancement of the Seebeck coefficient and the power factor is expected to further improve the ZT values of MoSe₂-based compounds by optimizing the doping elements.