This paper adopts a novel reflection amplifiers surface (RAS) to suppress the co-channel interference in the spatial domain. The RAS can reflect and amplify the electromagnetic wave with phase shifts by designing the reflection coefficients, which enables it more flexibly reconfigure the wireless propagation environment, and even suppress interference channel gain. In this paper, a transmitter and an interferer send the desired signal and interference to the receiver, respectively, and a RAS is placed to suppress the unknown interference. First, we design the reflection coefficients for optimizing the interference suppression ratio, and prove that when the number of reflection amplifiers is greater than the number of antennas at the interferer, the interference can be perfectly suppressed. Next, a capacity maximization problem is formulated to design the optimal reflection coefficients, and an iterative algorithm based on fractional programming and the convex-concave procedure is proposed to obtain the solution for this problem. Moreover, the closed-form expression of the maximal capacity is obtained in the strong interference power case. In addition, this paper shows the upper and lower boundaries of the maximal capacity and discusses what kind of the channel conditions achieve the upper and lower boundaries. Lastly, the above results are generalized to the multiple interferer scenario.