Abstract: In continuous wave radar tracking system,system error of range data is usally large,while range rate data has high precision.How to use rate data to determine orbit of a spacecraft is important.The theoretical analysis and actual computation results show that orbit determining by use of rate datas point by points has very poor precision and isn't valuable.The detailed study is given and the new nonlinear spline function method of orbit determining by use of rate data is presented.The actual algorithm and the error propagation formula are given Results show that the new method has high precision.The actual application shows that the new method is successful and widely valuable.