During the life cycle, the failure rate and repairable rate of repairable parts show uncertain characteristics. The birth and death process based on the determined failure rate and repair rate may not meet the demand forecasting of spare parts. By using interval gray numbers to represent the failure rate and repair rate of repairable parts, the gray state transition matrix is constructed. The spare parts demand forecasting model of repairable components based on gray birth and death process is built. The memoryless and existence conditions of steady solution of the process based on the gray birth and death process are studied. To some extent, the spare parts demand law with the uncertain information of the failure rate and the repair rate is revealed. The practical case study verifies the validity and practicability of the model.