In this paper, the assembly problem of circular-rectangular compound peg and hole parts in automatic assembly is proposed for the first time, and an automatic assembly method based on six-dimensional force sensor is summarized. Firstly, according to the quasi-static equilibrium condition, the contact states are summarized. It is concluded that there are 7 categories of 44 contact states in insertion stage and the forces conditions to maintain each contact state; Secondly, according to the analysis of the force/moment relationship, the jamming diagram is drawn, and the conditions of jamming are analyzed. Thirdly, the assembly strategy is discussed, including contact state recognition strategy, hole searching strategy, and pose adjustment strategy. Finally, combined with impedance control, the assembly theory is verified in ADAMS, and the assembly with minimum clearance of 0.1 mm is achieved.