The knowledge from extensive investigations showed that the non-linearity of the load and loading point displacement curve is mainly associated with crack propagation. Based on the linear asymptotic superposition assumption, the critical effective crack length is evaluated by analyzing the nonlinear characteristics of the load and loading point displacement curves of a series of the fracture tests. Furthermore, the unstable fracture toughness is determined by inserting the peak load and the critical effective crack length into a formula developed from LEFM. Based on the calculated results, the series K criterion of unstable fracture of concrete fracture is established. At last, an engineering example of cracking analysis of the upstream lock-head foundation of the ship-lift of the Three Gorges Project is given.