Deformation failures occur easily at multilateral wells connector in unstable formations. Therefore, a multilateral connection assembly is designed with 2 main prefabricated units, a template and a connector. These parts form a whole continuous mechanical structure after being locked by a tongue-in-groove trapezoidal rail. The designed gap and tolerance of this match satisfy the requirement for hydraulic seal of the connector. The analysis is processed on displacement and internal stress of the connection assembly under 3 separate formation pressures by using finite element numerical simulation. The results show that the deformation is very small and the maximum internal stress is less than the yield stress of N80 steel, which indicates that the connection assembly has a strong resistance to geo-mechanical forces.