As a significant research method, quasi-continuous high magnetic field (QCMHF) can meet scientific experimental requirements for higher magnetic intensity, longer flat-top pulsed width and lower ripple in the field of physics, biology and other scientific fields. In this paper, a QCMHF system designed by multi-objective optimization method is present to obtain the magnetic intensity of 70T with high-stability at Wuhan National High Magnetic Field Center. The magnet system consists of two coils, the inner coil is energized by the 100MVA/100MJ generator-rectifier power supply and the outer coil is powered by a 22MJ capacitor power supply. A protective circuit is set for short-circuit fault between the two power supplies. To improve the stability of the QCMHF, two decoupling methods are proposed and analyzed. In the hardware circuit decoupling scheme, a compensatory transformer whose mutual inductance counteracts the magnet’s mutual inductance is connected series in the circuit. In the software control decoupling scheme, the repetitive control strategy is applied to make the rectifiers output an extra voltage to offset the influence from the mutual inductance. Moreover, a self-adaptive closed-loop PI feedback controller is used to reduce the ripple of the QCMHF. The MATLAB/Simulink platform is adopted to establish the equivalent model of QCMHF system and the preconceived 70T/100ms flat-top pulse with ripple less than 200 ppm is generated.