Stimulated Brillouin Scattering(SBS) is an important nonlinear effect which is observed in optical fiber, the properties of SBS gain spectrum lineshape play a key role, for use in some applications such as spectrum analysis, microwave photonic filtering and optical fiber sensors based on Brillouin amplification. High signal to noise ratio (SNR) measurement data is impossible with current methods, limits analyzing the lineshape of SBS gain spectrum. A method for brillouin gain spectrum measurements in standard single-mode fiber by two narrow-linewidth laser is presented, using one as the probe signal to measure, and the other as the pump signal, sweeping a wide spectral span of around the probe signal. Benefit from narrow-linewidth laser and polarization pulling of SBS, high SNR measurement of brillouin gain spectrum is achieved. The brillouin gain spectrum is measured for different power levels of pump signal, the relation of power levels and relevant SBS parameters such as linewidth and gain profile is studied, particularly, the evolution of the SBS gain profile from Lorentzian to Gaussian as predicted by current theory is also experimental verification and analysis, show the SBS spectrum lineshape affected by the gain level involving different optical power levels of the stimulating signal. Experimental results show that with pump power up from 15dBm, the saturation is preset, so if directly measured the FWHM of SBS gain spectrum, its broadens as the pump power increases. This gaussian functional form of SBS gain spectrum could not be directly measured experimentally at high pump powers, linearizing the response is proposed, in order to get valid information of the convolution between the SBS spectrum and the measured signal spectrum, so the lineshape is confirmed to be gaussian. In the intermediate gain region, the SBS spectral shape does not fit well with current functions, lack of proper lineshape evolution model, so a mathematical model with Lorentzian function to the power of a variable value k was proposed, to resolve the difficult fitting problems of lineshape evolution from Lorentzian to Gaussian. Experimental results with excellent accuracy, proven an effective mathematical model of SBS gain spectrum lineshape, to describe the profile of SBS spectrum with different gain level. The use of SBS response for spectral analysis is proposed. Accordingly, the quality of the SBS based spectrum analysis is highly dependent on the optical power of the pumping signal, the SBS gain as filtering with Gaussian profile is used. The optical spectrum of 6314CA stabilized optical source with 0.2 pm resolution is obtained, show a great potential candidate for ultra-high resolution spectral analysis, will be a useful diagnostic tool for new generation optical network research and development (R&D).