A method and laboratory system for measuring the volume scattering function (VSF) of water approaching to 180° were developed based on an off-axis reflective optical path, which overcame the limitations of the current VSF system in the measurement of backscattering at small angle. The system adopted an off-axis parabolic mirror to separate the backscattered light at small angle from the incident laser, which reduced the blind area for measuring the backscattering at small angle. In addition, it could obtain the small angle backscattering signal within full azimuth angle. The polystyrene standard particles were used for the calibration and validation of the system. The results show that the calibrated measurement system can provide the measurement of the volume scattering function of suspended particles in the range of 173°-179.4°, with an angular resolution of 0.01°. The experimental values of the particulate VSF agree well with the theoretical value calculated from Mie scattering. This proves the accuracy and feasibility of the system in measuring the volume scattering function of water approaching to 180°.