In preheating stage after inflation, fluctuations of inflaton field may be enhanced exponentially by parametric resonance. The large and time dependent density inhomogeneity will produce stochastic gravitational wave background. We use lattice simulation to study the non-perturbative evolution of the inflaton field and the production of gravitational waves. We focus on axion monodromy inflation model and find that significant single-peak stochastic gravitational wave backgrounds are generated during preheating.