In recent years, the high-power single-frequency fiber lasers have developed rapidly, and they have been used in nonlinear frequency conversion and gravitational wave detection. The main factors limiting the output power of single-frequency fiber lasers are the nonlinear effect and transverse mode instability (TMI) effect. In general, large-core fibers can mitigate nonlinear effects while small-core fibers help to suppress the TMI effect. Owing to the core diameter varying in the longitudinal direction, tapered double clad fiber (T-DCF) is a promising solution to simultaneously suppress the nonlinearity and TMI effects. In the present study, we have fabricated a piece of 2.2-m-long Ytterbium-doped T-DCF. The core diameter and the cladding diameter of this fiber vary gradually from 30.3 μm to 49.3 μm and from 245 μm to 404 μm, respectively. Using this homemade fiber, we constructe an all-fiberized single-frequency master oscillator power amplifier system, which is pumped by laser diodes with a central wavelength of 976 nm. The seed of the system has a central wavelength of 1064 nm, and output power of 30 mW. The T-DCF is coiled on a piece of cooling plate, whose output end is cleaved at a 8° angle. The laser is output to free space and collimated by a free-space collimator. After the collimator, dichroic mirror is utilized to strip out the residual pump power for measuring power, spectrum, time-domain signal and beam quality. The output power increases linearly with the pumping power increasing. When the pumping power is 502 W, the output power reaches 400 W. And there is no stimulated Brillouin scattering (SBS) nor TMI under the power level. The corresponding slope efficiency is 81.7% while the M² is measured to be 1.29, exhibiting the single-mode output characteristic of the system. When the output power is further increased to 418 W, the TMI effect is observed, which limits further the power scaling of the single-mode output. To the best of our knowledge, this is the highest output power of single-frequency fiber laser based on home-made gain fibers. The results indicate that T-DCFs can simultaneously suppress the nonlinearity and TMI, thus providing a useful reference for further power scaling of single-frequency fiber lasers. Higher output power is expected by optimizing the parameters of T-DCF and the structure of system.