Pressure vessels are widely utilized in many areas of industrial production and daily life for medium storage, which causes performance degradation in pressure vessels, such as crack and corrosion, and lead to serious safety and financial consequences. Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) is a kind of guided wave-based tomography method which is suitable to evaluate structure integrity of pressure vessels. In this article, the effect of liquid level on guided wave propagation and imaging results of RAPID algorithm is investigated, and an optimal baseline matching method based on amplitude variance is proposed to improve the imaging accuracy of RAPID algorithm with liquid-contained condition. The attenuation effect of liquid on guided wave amplitude is investigated. The damage signals are matched with baseline signals recorded at different liquid levels, and the effect of liquid on RAPID algorithm is discussed based on the results. The experiment of image reconstruction for pressure vessel using the optimal baseline matching method based RAPID algorithm is conducted as well. The experimental results show that the optimal baseline matching method can effectively select the best baseline signal, and the reconstructed images can accurately locate the defects on pressure vessels with considering the change of liquid level.