Formal verification has been widely adopted to check logic correctness. One of the challenges in formal verification is how to quickly reach a formal proof for a user-specified property. This is especially difficult when the property involves word-level reasoning. In this work, we propose to augment the target property with additional conjectures automatically learned from simulation traces. The conjectures are generated by a reinforcement learning model, which dynamically expands production rules according to observations from simulation. Experiments show that our property strengthening method achieves notable speed-up on multiple verification tasks, including sequential equivalence checking and word-level property checking.