Abstract: The mechanical model of shape memory alloy composite diaphragm based on Si substrate was established by combining static equilibrium equations, geometric equation and physical equations by the method of the mechanics of materials to consider the coupled action between the shape memory alloy and Si substrate under thermal loads. The material nonlinearity of the shape memory alloy film is also considered. The actuation performances of the SMA composite diaphragm were simulated and discussed for one whole thermal loading cycle, which verifies that large actuation forces and displacements can be produced in a narrow temperature range due to the phase transformations of shape memory alloys. Additionally, the maximum deflection of the composite diaphragm varying with the thickness ratios of the Si substrate to the shape memory alloy film was studied. It is found that the maximum deflection of the SMA composite diaphragm decreases with the increasing of the thickness ratios. And the mechanical model established in this paper can describe and predict the actuation performances of the SMA composite diaphragm much more accurately when the thickness ratio is larger than five.