Abstract: In this paper, a series of experiments on the chaotic evolution of pressure drop oscillations (PDO) under rolling motion were designed and performed. The chaotic characteristic is analyzed based on maximal Lyapunov exponent (λ). It is concluded that the chaotic evolution route can be divided into 3 regions. In the thermal-hydraulic stable region, it is a stochastic low-amplitude vibration and PDO cannot happen. In the frequency lock oscillation region, PDO is forced to keep the same frequency as rolling motion and forms quasi-periodical oscillation with increasing λ. In the rolling effect weakening region, PDO with natural frequency can generate reverse flow. λ reaches the maximum value and shows strong chaotic characteristics.