The Brillouin spectroscopy technique based on Virtual Image Phased Array (VIPA) is a high-resolution spectral detection technique. It offers advantages such as real-time monitoring and high sensitivity. Currently, this technique primarily relies on the mapping relationship between the Free Spectral Range (FSR) of VIPA and the pixel to obtain frequency information. However, variations in the incident light angle can cause fluctuations in the FSR. Traditional measurement methods are based on the assumption of constant FSR and do not provide real-time calibration for FSR changes caused by variations in the incident angle, resulting in measurement errors. In this paper, we address this issue by analyzing the characteristics of Brillouin scattering and the interference principle of VIPA. We propose a real-time calibration method for FSR based on the Brillouin frequency shift of a single-mode fiber and a corresponding calculation method for pixel-to-frequency mapping. This method utilizes the Brillouin Stokes and anti-Stokes frequency shifts of a single-mode fiber as references to reduce frequency measurement errors caused by FSR fluctuations due to changes in the incident light angle. Experimental results demonstrate the rapid measurement of micro/nanofiber Brillouin frequency shift in different refractive index environments at a wavelength of 532 nm using this method.