It is the key problem to ascertain rock engineering parameters in numerical calculation. Rock engineering parameter accuracy affects the outcome of numerical calculation. However， it is difficult to get the true rock engineering parameters because of the scale effect in laboratory and insufficient quantity of sampling. Many factors can be considered by back analysis method，which can get the rock mass parameters accurately. For the positive and negative analysis method， the planned back analysis parameters are postulated at first， and the displacement or stress of rock mass is obtained by positive and negative analysis. By comparing the displacement or stress of rock mass with the monitoring data， the back analysis parameters are adjusted step by step until the displacement or stress of rock mass approximates the monitoring data. Finally， the right rock mass parameters are obtained. By abroad maritime tunnel of mudstone， rock parameter back calculation is carried out on the basis of the monitoring data. The actual rock engineering parameters are obtained by analysis and comparison，which lays a foundation for ascertaining tunnel initial support parameters.