For the precise targeting in inertial confinement fusion (ICF), the requirements for beam transformation, beam shaping and beam control are harsh. The digital control of key technologies online can realize the automatic calibration of the laser system, of which frequency conversion is a crucial part. Based on the reversible algorithm of Fourier transform and Runge-Kutta method without any iterative computation, the inverse problem of type “I+II” KDP frequency conversion is studied in detail on the final optical assemble (FOA) at SG-II upgrade facility. And, some numerical calculation results are also presented. Finally, compared with experimental datas at SG-II upgrade facility, the correction of corresponding theories and the stability of algorithm are verified during the derivation.