This paper proposes a stabilized multichannel random access protocol based on slotted ALOHA for relay deployed cellular networks. To ensure the stability of random access, the proposed protocol dynamically controls the number of random access channels in a BS and a RS and the retransmission probability of the random access packets under heavy load conditions. A mathematical formula is also developed that derives an optimal partition ratio of the shared random access channels between a base station and a relay station without and with capture effect. Numerical results show that the proposed protocol can guarantee the required utilization and delay even in high offered load, which otherwise can cause bistable problem of slotted ALOHA.