In this paper, we propose a physical-layer secret key generation scheme for multi-antenna legitimate nodes with the help from multiple untrusted relays with single antenna. The relays' actions conform to the rules but they passively eavesdrop the information. Different from most previous key generation schemes, where a key is generated based on a channel coefficient, in the proposed scheme, we use the linear combination of channel coefficients to generate a key. Simulation results show that the rate leaked to the untrusted relays is low and the secret key rate is therefore high. We also point out that the relay communication can achieve higher secret key rate than the direct communication in several distance ranges.