Raman magnetic resonance (RMR) spectroscopy in weakly coupled two-spin system (C-H spin system in CHCl3 molecule) has been investigated in detail by product operator formalism and RMR experiment, where the irradiated nucleus is proton and the detected nucleus is carbon. The theoretical results predicted by product operator formalism are in good agreement with the RMR experiment in various radio-frequency offsets and irradiation strengths applied to proton, indicating that product-operator formalism has advantages over the perturbation approximation-method and eliminates limitation of the approximation method that can be only used to describe the RMR experiment under the case of weak radio frequency irradiation. The studies also show that product operator formalism is a powerful tool for examining the RMR experiments in complex spin systems. It is found that the multiple quantum transitions are much easier to be observable with small radio frequency offsets and higher signal-to-noise ratio is obtained with the moderate irradiation strengths in RMR experiment.