Symbolic execution faces the challenge of generating valid inputs when analyzing the program with complex input formats. Token-based symbolic execution can partially tackle this challenge but is still doomed by the difficulty of passing input checking and failing to analyze the code after input checking. We propose \lase, an online input grammar synthesis aided symbolic execution method, to generate valid inputs for improving the effectiveness of symbolic execution. Inside \lase, we propose an input grammar-oriented search strategy and a token-level grammar synthesis method. The search strategy selects the paths to cover more syntax rules in priority. The token-level grammar synthesis improves the synthesized grammar’s precision and completeness while ensuring efficiency. The experimental results on real-world parsing programs with complex input grammars demonstrate that \lase\ can improve the coverage of parsing code and generate more valid inputs to improve the coverage of functionality code significantly. Furthermore, compared with the state-of-the-art grammar synthesis methods, the grammars learned by \lase\ have better precision and recall on most benchmark programs.