For a digital signature scheme, unforgeability and non-repudiation are two main security requirements. In 2017, Galbraith, Petit and Silva presented GPS signature, an efficient isogeny based signature with a proven unforgeability. In this paper, we present a successful key substitution attack on GPS signature which threaten the non-repudiation of GPS signature. We also suggest how to prevent key substitution attack in general as well as our attack in this paper. We also present an example of our attack using Sage to illustrate isogenies of elliptic curves and our attack.