In order to overcome the shortcoming of small and untunable shear difference in the existing parallel splitting prisms, and expand the application scope of the parallel splitting prisms, a composite parallel splitting prism is made by one iceland crystal prism and two triangle glass prisms with adjustable distance. The composite prism has the following advantages: low cost, shear difference tunable in a large scale, excellent splitting ratio and total transmission ratio. The changes of tuning scale, the e light and o light splitting ratio and the total transmission ratio without antireflection film coating of the composite prism with the the change of structure parameters including the distance between the two glass prisms and the glass prism angles are analyzed theoretically. Computer simulation results are given too. Based on the results, an experimental sample is designed and fabricated. When the distance between two glass prisms changes from 15 mm to 180 mm, for the transmitted light via Iceland crystal prism with 3 mm shearing distance, the shearing distance can be tuned between 1.4~50.2 mm. And the splitting ratio of e light and o light is less than 1.1, the total transmission ratio is higher than 80%.