A widely tunable distributed feedback (DFB) semiconductor laser with constant power and narrow linewidth is fabricated. The laser chip is based on the asymmetric phase-shifted DFB structure, which can effectively narrow the bandwidth of the output. By precisely controlling the temperature and driving current, the dynamic characteristic of internal carriers and the refractive index of the material can be controlled effectively, so wide-band tuning wavelength and constant output power are obtained. The wavelength tuning range is over 3.5 nm, the laser power is 7.4 mW, the linewidth is about 220 kHz and the side mode suppression radio is 52.7 dB. This laser has wide application prospects for tunable diode laser absorption spectroscopy (TDLAS).