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We present a procedure for determining the linewidth enhancement factor (α factor) of semiconductor lasers under continuous-wave operation with a proper correction of thermal effects. Gain spectra are measured by the Fourier series expansion method, and the variations of refractive index are determined by the mode wavelength shift and Kramers-Kronig relation, respectively. The α factors are measured in a wide wavelength range from 1170 to 1320 nm for an InAs/GaAs quantum dot (QD) laser with a cavity length of 440 μm. The α factors of 0.42 and 0.79 are obtained at gain peak wavelength of 1210 nm for the excited state at the injection currents of 40 and 60 mA, and the corresponding values of 1.97 and 3.97 are obtained at a wavelength of 1285 nm, respectively. The results show a relative low α factor at the gain peak position of QD excited state. We also calculate theoretically the optical gain and α factor for the QD laser, and compare with the experimental results. The results indicate that high injection current will result in higher α factor due to the appearance of asymmetry gain spectrum.