In the PrF3 crystal, the lowest crystal field state of Pr3+ ion is a single state, which has no contribution to the susceptibility and the Verdet constant of the crystal in first-order effect. The secondary crystal field-effect admixes the state with other states and makes the state have a great contribution to the magnetic properties of PrF3 crystal. In this paper, the effect of secondary crystal field-effect on the magnetic and magnetio-optic properties of PrF3 crystal is presented. Furthermore, the effective field of exchange interaction between the ions is investigated, which has the form is Hin=(1.9-0.02556 T)×10-5M. By means of the effective field, the inverses of the magnetic susceptibility and Verdet constant of PrF3 crystal as functions of temperature are calculated, and the results are in good agreement with the measured ones. The results show that the crystal field's second-order contribution and exchange interaction between ions shouldn't be ignored.