Force Limit Method can decrease the over-testing problem associated with traditional vibration testing by dual control of the force and acceleration. Force specification is difficult to estimate, especially for equipment vibration test that exists the source definition problem. This paper demonstrates a modified complex two-degree-of-freedom method adapted to fixed boundary conditions of the mounting structure. Some methods are also given to acquire the mass parameters of source and load, which are critical to the reasonably conservative force specification. Practical examples have been performed with the modified method to verify its validity, which provides reference for force specification determination.