Abstract: In order to analyze the instability mechanism of rock beam under multi-jointed rock, a multi-jointed rock beam model was established by using 3DEC. The destabilization process and instability forms of multi-jointed rock beam were simulated and analyzed. The results show that when the rock mass of the rock beam is odd, the rock beam is unstable and the four-hinged arch structure is applied. When the number of rock blocks is even, rock beam is three-hinged arch structure force; the failure of the rock beam mainly includes the slip of the middle rock mass and the cracking of the joint surface at the hinge point; due to the role of horizontal thrust, joint surface cracking occurs only at the hinge point; the degree of joint cracking reflects the degree of joint surface cracking; when the joints are aligned, the section of rock mass is rectangular; when the joints are staggered, the section of the rock mass is an isosceles trapezoid; when the joints are staggered, the stability of the rock beam is higher than that of the aligned joints.