The tunable optical power splitter is a key optical component used to dynamically control its optical power at its output ports, which has a wide application in many optical fields. A novel approach to controllable optical power splitter is proposed in this work, which is based on thermal expansion effect resulting in width variation of air gap at the branching point of Y-branch waveguide. The thermal expansion profile and the optical performance are simulated by using the finite element method (FEM). The simulation results show that the tunable optical power splitter can exhibit good performance such as large dynamical range, low dependence of operation wavelength and its optical polarization. In addition, the proposed splitter with a simple structure can be easily designed, fabricated and controlled, which is very useful for potential application in integrated optical system.