It’s very important to predict the squeezing level of a tunnel for designer, so it’s necessary to study the critical strain of squeezing state. The use of Q and RMR can’t get ideal effect in weak rock mass, so GSI is introduced to replace the Q and RMR. The formula of critical strain is deduced based on other research results. An engineering example is used to demonstrate the formula and the calculated result is consistent with the practically observed result. In the end, according to Hoek-Brown strength criterion, the analytical formula which expresses the critical strain in squeezing state of a circular tunnel is derived in hydrostatic pressure state. The side-pressure coefficient is nearly equal to 1 in deep buried tunnel, so the analytical solution can be used to determine the squeezing state of tunnel.