Consider the following fractional Schrödinger equation
where λ>0, s∈(0,1), N>2s, 2<q<p<2∗s (2∗s=2NN−2s), P∈L∞ is positive, Q∈L∞ may be positive, sign-changing or negative, V is steep well potential, and V0∈L∞. When λ is large, the existence of nontrivial solutions is obtained via variational methods. Furthermore, if V(x)≥0, concentration results are also obtained. In particular, the potential V is allowed to be sign-changing for the existence.