The research results of the intrinsic birefringence of the circular-core side hole fiber are reported. On the basis of the study on the production mechanism and calculation principles of the intrinsic birefringence of the side hole fiber, the finite element method is used to analyze the interior stresses distribution and the birefringence values of the circular-core side hole fiber. The research results indicate that the geometrical birefringence of the circular-core side hole fiber can be neglected, and the birefringence mainly relies on the stress birefringence; the stresses distributions in the core and cladding field have changed greatly because of the presence of the side holes. The conclusion is put forword that the fast axis of the circular-core side hole fiber is parallel to the link line of the two holes. The influence of different physical dimensions of the holes on the intrinsic birefringence of the side hole fiber is also discussed and the results show that the intrinsic birefringence of the circular-core side hole fiber with round holes increases exponentially with the angle of side holes. The birefringence can be improved through increasing the radius of the side holes and reducing the distance between the core and the side hole.