This paper proposes a physics-based cruising speed model for biomimetic robotic fish propelled by the passive plastic foil actuator. Inspired by biological body and fins, a plastic foil with suitable rigidity and innovative structure are designed to acquire better propelling effect. The model captures the speed based on the analysis of the interaction between the tail and the water which considers the hydrodynamics. Experimental results have shown that the proposed model is able to predict the steady-state cruising speed of the robotic fish under a periodic actuation input. Since most of the model parameters are expressed in terms of fundamental physical properties and geometric dimensions, the model is expected to be beneficial in optimal design of the robotic fish.