To investigate the small signal stability of multi-infeed HVDC system, a multi-input and multi-output interconnected model with variable speed is established. The model is divided into two separate subsystems which are composed of LCC-HVDC and the rest of the power system. When two open-loop modes of two subsystems are close to each other, a special condition of near strong mode resonance (NESMOR) will take place, and strong dynamic interactions occur between subsystems. Under the NESMOR condition, one of the closed-loop modes is on the right of the corresponding open-loop mode and the damping of the closed-loop mode will be wicker, thus the small signal stability of the system will degrade. A method is proposed to predict the positions of the closed-loop modes based on the open-loop state space equations,also an index and analysis process of evaluating the stability of the system are given. In addition, the impact of the length of the tie-line between the infeeds is studied. Finally, simulation study of example power system is presented to verify the accuracy of the proposed method.