A rate-shift-based integrated system is proposed to increase the throughput of the joint radar-communications systems. We show that the transmission rate has a time-varying bound and can be maximized with each transmission bit having a different rate. Then, we derive the theoretical throughput of the rate-shift signal, which doubles that of the constant rate signal. For achieving the double throughput, a rate-shift algorithm is proposed to generate the transmission rates of rate-shift signal. The corresponding transmitter and receiver are designed without any additional computational complexity. The numerical results show that the throughput of joint radar-communications systems can be significantly improved by shifting rates.