The high power single frequency laser at 507.4 nm is proposed based on the high efficient frequency doubling technology of a 1014.8 nm room-temperature fiber laser amplifier. High efficient frequency doubling is realized in a high gain ring cavity with a normal incidence lithium triborate crystal. The double frequency efficiency is up to 61.5% and the output power is up to 3 W. The experimental data and simulated results of the input-output characteristics of the frequency doubling are coincide. The double frequency efficiency is insensitive with the input power at the peak working point when the input power of infrared laser is more than 4 W. The power fluctuation in root mean square is 1.7% in 1.5 h. The high power single frequency laser at 507.4 nm can be both used to generate the 253.7 nm ultraviolet laser for laser cooling of neutral mercury atom by barium boron oxide crystal, and to detect the transition of ytterbium atom from 1S0 to 3P2 state, and related experiments.