The mean wavelength dependence on pump power in an Er-doped superfluorescent fiber source of double-pass backward configuration was studied. Data presented shows that mean wavelength variations with pump power can be reduced to -9 ppm/mW for selected pump powers by using an Er-doped fiber filter. And the total mean wavelength instability of the fiber source is less than 33 ppm(peak-to-peak) over the -40 ℃ to 60 ℃. The bandwidth reaches 17 nm. The pump power is 55 mW at 974.2 nm and the output power reaches 5.83 mW with 5-m Er-doped fiber.