Hybrid automatic-repeat-request(HARQ) scheme can be incorporated with the linear precoder in multi-input multi- output(MIMO) transmission to ensure highly reliable communications. To fully utilize type-I HARQ diversity gain especially in the slow fading channels, we propose the optimal design principle of linear precoders, whose column vectors are orthogonal to each other correspondingly. Simulation results demonstrate the effectiveness of these precoders in reducing the detection bit-error rate and increasing the unified throughput.