We study the stability theory of solitary wave solutions for a type of derivative nonlinear Schrödinger equation i∂tu+∂x2u+i|u|2∂xu+b|u|4u=0,b>0.The equation has a two-parameter family of solitary wave solutions of the form uω,c(x,t)=exp{iωt+ic2(x−ct)−i4∫−∞x−ct|φω,c(η)|2dη}φω,c(x−ct).Here φω,c is some suitable function and −2ω<c≤2ω. The stability in the frequency region of −2ω<c<2κω (for some κ∈(0,1)), and the instability in the frequency region of 2κω<c<2ω were proved by Ohta (2014). Recently, in the endpoint case c=2ω, the instability of uω,c was proved by Ning et al. (2017). Then the stability and instability region has been established except the degenerate case c=2κω. In this paper, we address the problem and prove its instability in the degenerate case.