Through turning experiments, machined residual stress and micro-hardness of hardened layer on all samples were measured at different stages of tool wear, changing characteristics and distribution of machined residual stress were understood, and then how and why tool wear affecting turning machined residual stress were analyzed. Experimental results show that the turning machined residual stress changes slowly in the former stage of wear, and rapidly changes at a later stage. The residual stress is from the trend state of pull-compressive stress mixture transition to compressive stress, and stress change rate does not coincide to tool wear curves.