FeSe_1-xTe_x superconductor has excellent physical and chemical properties. However, anisotropy of the critical current density (J_c) and lack of strong flux pinning still hinders it from practical applications. In this article, Fe_1-ySn_ySe_0.4Te_0.6 crystals were grown to study Sn doping effect on the improvement of the superconducting performance. Compared with the undoped, the J_c and the irreversible field H_irr are significantly enhanced for Sn-doped crystals with y=0.08. In the temperature range of 4.2-8 K and the magnetic field up to 7 T, the J_c is increased by 4 times, at the meantime, the J_c anisotropy parameter, γ = J_c^H||c/J_c^H⊥c is reduced significantly. Structure and microstructure analysis shows that doping Sn in FeSe_0.4Te_0.6 reduces the crystal anisotropy and introduces secondary phase nanoparticles, which can be regarded as the main reason for the reduction of the J_c anisotropy and the significant enhancement the magnetic flux pinning force of FeSe_0.4Te_0.6 superconductors. Due to the existence of intrinsic pinning or the new pinning mechanism introduced by nanoparticles, the pinning behavior of the FeSe_0.4Te_0.6 in H⊥c or in the higher doping level sample (y=0.08) significantly deviates from the δℓ-type point pinning behavior which was often observed in the in-plane pinning for undoped FeSe_0.4Te_0.6 crystals.