The study of countermeasures for hardware fault attack in multi-prime RSA cryptosystems is very important for applications such as computer network and smart cards. In this paper, an efficient countermeasure method is proposed for the FPGA-based multi-prime RSA systems. The proposed method can survive the attacks [27, 30] that broke the previous methods [5, 33]. Furthermore, by using a simple operation and small wordlength parameters, the proposed method is very efficient in terms of hardware resources and speed. In order to verify the effectiveness of the proposed method, the FPGA implementation and testing in attacking environment are carried out for several two-prime and three-prime design examples.