This paper proposes a new model to calculate anchorage length for the compression anchor. Then the analytic solutions for the effective length and the critical length are derived according to the model. The analysis to the solutions show that the larger the rock mass’s elastic modulus, cohesion and internal friction angle, the smaller the anchorage length. Besides, the larger the load, the greater the anchorage length. However, the poisson’s ratio of rock mass nearly has no influence on the anchorage length. It is most important that the two calculation length show great agreement with present codes or specification.