The three-dimensional small angle measurement system based on Fresnel biprism was an improved autocollimation method which replaced the single mirror by two mirrors with a certain angle. It measured the space angle change through solving the mathematical relationship between the reflected spot displacement and the mirror rotation angle. The traditional modeling method always predetermined a rotation order to indirectly establish the mathematical relationship between the state of the object space angle change and the rotation angle, which had not one result for the predetermined rotation order was random. A new modeling method was proposed which directly established the mathematical relationship between the initial object state and the final state after rotation, which didn′t need to predetermine a rotation order making one rotation state having only one mathematical relationship. This method was objective, direct and could be used to measure bigger range of three-dimensional angle. Finally, the correctness of this modeling method by using simulation calculation was verified.