The influence of modulation rate on energy efficiency (EE) and the various system parameters on the optimal EE for a sourcerelayterminal communication link using amplifyandforward (AF) protocol and MQAM are studied when bit error rate (BER) is prescribed. First, the transmission power consumption is calculated, then the total power consumption can be obtained when circuit power consumption and demodulation energy consumpton are taken into account. Finally, the expression of EE is derived and the influence of modulation rate on EE is studied. Through simulation results we find that when the modulation rate increases, the energy efficiency first increases and then decreases. When the error rate increases, the optimal EE strictly increases, the optimal modulation rate strictly decreases. When increasing the circuit power consumption, the optimal EE strictly decreasing and the optimal modulation rate strictly increasing. When the initial energy consumption increasing with ML demodulation receiver, the optimal EE strictly decreasing and the optimal modulation rate strictly increasing.