Abstract: In analysing the deformations of Engineering monitoring nets,the most important thing is to find a mathematical model which is similar to the true deformations.1n this paper,through calculating a monitoring net of large dam,the author indicates that generalized inverse adjustment(free net adjustment) is only one of the many mathematical models.Since the deformations arc affected by different factors,generally speaking,the selected model can not be perfectly similar to the true one.The deformations △x calculated in the generalized inverse adjustment have △x^T△x⇒min property.For a monitoring net with some moved points,△x calculated are smaller than the true value.The result calculated in this paper indicates that in some cases the differences are not negligible and hence it is not safe for monitoring engineering projects.We should be very cautious in calculating the deformations of the points of monitoring nets.