This paper presents a cooperative transmission scheme for cooperative multi-relay high-speed railway mobile communication system, where fountain codes and mutual information accumulation (IA) method are adopted. The spreading codes are used to separate and detect the signal with interference from multiple relays, and the signal detection process and information receiving strategy at the destination are designed. Then we analyze the performance of the system in terms of the transmission rate, where the probability density function of the transmission rate of IA is derived, as well as the corresponding average transmission rate. For comparison, the traditional energy accumulation (EA) based receiving scheme is also analyzed. Results show when spreading codes are not orthogonal, if the signal to noise ratio (SNR) is relatively low, the performance of IA-based information receiving method is worse than that of EA-based one, and if SNR is relatively high, the performance of IA-based information receiving method is superior to that of EA-based one.