The condensation of ethyl bicyclic phosphite (1-ethyl-4-phospha-3,5,8-trioxabicyclo-[2,2,2]-octane) and dimethyl methylphosphonate has been investigated using self-made ionic liquid [Bmim]BF4 as catalyst under solvent-free conditions. The effects of process parameters, such as molar ratio, reaction temperature and catalyst on the yield were studied respectively from the orthogonal experiment.The suitable synthesis conditions as follows: the molar ratio of ethyl bicyclic phosphite and dimethyl methylphosphonate is 1:1.6,reaction temperature is 185℃,reaction time is 16h,catalyst dosage is 0.4% of the amount of all. The cyclic phosphonate ester was obtained in this condition with the yield 98%. TG test under pure nitrogen showed it had high residual char rate at 500℃.