Objective To study the stress distribution and biomechanical stability of the long-type composite locking plate already used in clinic practice and the novel short-type composite locking plate for treating Sanders type IIa, IIb and IIIab calcaneal fractures. Methods The three-dimensional (3D) models of Sanders type IIa, IIb and IIIab calcaneal fractures were established, and the force conditions of calcaneus in neutral standing position and under 20°dorsal extension were simulated. By referring to the physical form of human specimens, 500 N vertical axial load was applied, so as to study the displacement and relative displacement of the fracture block under the force, and the force conditions of bone tissues and internal fixation were analyzed. Results For Sanders type IIa, IIb calcaneal fractures treated with long-type and short-type composite locking plates, the stress concentration positions of the plates and calcaneal fractures were basically the same. The maximum stress difference of the two plates for fixing calcaneal fractures with the same type was smaller than 5 MPa, and there was no significant difference in the maximum displacement of the fracture models. For Sanders type IIIab calcaneal fractures treated with long-type and short-type composite locking plates, the maximum stress concentration occurred in the forearm of plate screws, indicating the risk of metal fatigue. The maximum stress difference was up to 12 MPa, and the maximum calcaneal displacement was up to 9 μm. Conclusions The long-type and short-type composite locking plates showed no significant differences in treating Sanders type IIa, IIb calcaneal fractures. For fixing Sanders type IIIab calcaneal fractures, the long-type composite locking plate was superior to the short-type composite locking plate.