The spatial resolution of BOTDR (the Brillouin optical time domain reflectometer) based on pulse pump in fiber could reach meter order. The analytic relationship between Brillouin backscattering spectrum and stress gradient is set up to investigate the influence of the stress gradient within the resolution on the measurement accuracy of Brillouin frequency shift. The numerical results show that the increasing stress gradient results in the linear increasing of the Brillouin frequency shift, the nonlinear decreasing of the peak and the flattening of Brillouin spectrum. With the spatial resolution of 10 m and the stress gradient of 100 με/m, the measurement error of Brillouin frequency shift is 2.04 times larger than that without the stress gradient. Based on pulse modulation of lightwave with linewidth less than 1 MHz and BOTDR with balanced coherent detection, the Brillouin spectra and stress along the test fiber are experimentally measured at different stress gradients with the sensing fiber fixed on a cantilever beam. The Brillouin spectra of the different stress gradients are obtained from the experimental data by the least squares fitting. Experimental results are in agreement with the theoretical prediction.