Photocurrent power spectral density function of laser heterodyne detection is obtained by the statistical theory and Wiener-Khinchin theorem. For a short-range distance heterodyne system without considering atmospheric turbulence, we observe the relations between the photocurrent spectral line distribution and the laser linewidth, the intermediate-frequency signal, and the propagation delay time of signal light relative to local oscillator light. Theoretical formula of photocurrent power spectrum in relevant papers is revised to eliminate the effect of laser linewidth. Onedimensional probability distribution model of phase noise caused by laser linewidth is built based on the signal and noise theory. Accordingly we establish a mathematical model of limit detection accuracy based on laser wavelength, detection distance, and laser linewidth, which indicates the minimum detectable amplitude of heterodyne system. According to the numerical results, we find that the distribution of photocurrent spectral line intensities is greatly dependent on the relation between delay time and coherent time. And the minimum resolvable displacement increases with the detection distance and laser linewidth increasing. When the optical limited displacement resolution is 0.266 nm with a laser wavelength of 532 nm, a laser linewidth is 1 kHz, and a detection distance is 100 m. Experimental data in relevant papers agree well with the theoretical derivations. Our findings show that the research of displacement resolution might provide a quantitative reference for the theoretical research and engineering application of short-range heterodyne resolution.