Abstract: To deal with the optimum diameter of pipe roofs applied in underground engineering,an algorithm for the sensibility analysis of pipe roof displacement based on the incremental motion equation was developed.Two case studies on urban underground engineering with shallow overburden were conducted.The effects of pipe roof length,support span,etc.on the optimum diameter were analyzed.It was shown that the optimum diameter depended on the load bearing or load transferring function of pipe roofs and was affected by some factors,such as the pipe roof length,support parameters,construction technique,etc.