In order to meet the application requirements of deployable mechanism in agricultural machinery, aerospace and other fields, a deployable mechanism based on tetrahedral unit was proposed to solve the problems of low folding rate and unadjustable orientation of some existing mechanical arms. Firstly, a single degree of freedom (DOF) tetrahedral unit mechanism was introduced, which had good expansibility and high rigidity. Based on the screw theory, the DOF of tetrahedral unit was analyzed. Secondly, a multiple DOFs deployable mechanical arm mechanism was constructed by combining several tetrahedral units. Taking the deployable mechanical arm mechanism composed of two tetrahedral units as an example, the position, orientation and speed of each node were solved based on geometric method, and the motion law of the mechanism was obtained. Then, the theoretical analysis results were verified by ADAMS simulation analysis, and the theoretical values of the position and velocity of the end node were highly consistent with the simulation values. Furthermore, the statics and modal analysis were carried out by using Workbench simulation software to demonstrate the high rigidity and stability of the deployable mechanical arm mechanism. Finally, a prototype of machining and assembly principle was built and the deployable experiment was completed. The results showed that the deployable mechanical arm based on tetrahedral unit had the characteristics of multiple DOFs, adjustable orientation and large folding rate.