Abstract: Some parameters of the rock mass, such as elastic modulus, strength parameters, etc.,are very important to underground tunnel design and construction. Lots of cost are needed if situ-test is done. In this situation, application of displacement inverse analysis theory is very important for real engineering. But unfortunately most uncertain factors cause unavoidable random result for field monitoring, sometimes cause mistake. Factors obtained by inverse analysis on absolute displacement value of situ-test may be error. Such error can be avoided by relative displacement inverse analysis method. In the paper stochastic inverse analysis based on relative displacement method is used in a underground tunnel to calculate elastic modulus factor, and obtained a quite good result.