By using a new type pure rolling fatigue test rig with three contact points for silicon nitride bearing ball specimen, the contact deflection angle of specimen with three contact points is calculated. The effects of the center height and test ball radius on the specimen contact deflection angle are analyzed. The analysis indicates that the span of contact deflection angle is 0-0.181° while the center height is 1.2mm and the design radius is 6.35mm and the specimen radiuses change from 4mm to 8mm. The more contact deflection angle is, the more variation rang of test ball radius is. The contact deflection angle can be less than 0.2° while the center height is optimized. The contact deflection angle has a direct influence on the stress error of the specimen. It is less than 1% while the contact deflection angle is less than 0.2°. The research provides a theoretical basis for optimization of test rig structure size and increase of specimen’s extension and improvement of test precision.