In the presence of sea-surface multi-path，due to the interference of multi-path reflection, the measurement of a low elevation target by monopulse is often greatly affected. So off-axis tracking technique was adopted generally. By means of the analysis based on the model of multi-path reflection environment，both specular reflection and diffuse reflection were considered. Expression for the average bias in the mono-pulse ratio due to specular and diffuse reflections and the noise in the radar channel was derived. Algorithm was combined with bias compensation so as to adapt to the elevation measurement of off-axis tracking targets in the presence of sea-surface multi-path. This is aimed to avoid the disadvantage of both techniques employed here. Throughout the simulation in some scenarios, the results show that the method combining the two algorithms together can improve the precision of low-altitude target elevation measurement. Then it was applied in the extraction of postal data of off-line cruising flying object's elevation measurement, and a comparison was made with real time data elevation measurement by actual radar system. The results indicate that this method is effective and feasible in the off-line elevation extraction and it can remarkably improve the precision of elevation measurement