A method for measuring laser linewidth at Fourier limit based on time resolution and Fourier transform is proposed, whose resolution is limited by Fourier transform. In experiments, the linewidths of a fiber laser and a semiconductor laser are measured by the time-resolved method, which are compared with those by the traditional radio frequency (RF) spectral methods. The linewidth measurement results of these two lasers under different integration time show that the proposed measurement method at Fourier limit has smaller measurement errors than those by the RF spectral analyzer. Moreover, the acquisition of spectral information by the time-resolved method has the advantage of real-time acquisition.