In this paper, we propose a new transmit power diversity scheme for contention resolution diversity slotted ALOHA (CRDSA) under total transmit power limitation in satellite networks. In the proposed scheme, the total transmit power of each device within a frame is considered to be the same, and allocated to multiple packet replicas, and the transmit power of each packet replica can be different, which enables the transmit power diversity. The threshold based capture model is employed at the gateway demodulator, and the recovery-error probability based on this model is derived. The threshold of capture and transmit powers of packet replicas are also jointly optimized to maximize the achievable sum rate (ASR) of CRDSA under total transmit power limitation. Simulation results show that the proposed scheme is more than 50% higher than the conventional one in terms of normalized ASR.