The entanglement of two coupling spin 1/2 particles induced by outside magnetic field was studied under Heisenberg picture. By solving the differential equation group of systemic polarization vector varying with time using numerical method, the rule of polarization vector varying with time was obtained. Calculation results reveal that the un-entangled state of two coupling parallel spin 1/2 particles with total polarized vector z component being -2 could translate to entangled state under suitable outside magnetic field condition. The best parameters of outside magnetic field were obtained, and a new method was put forward to judge whether the system of two coupling spin 1/2 particles was in entangled state based on the change of polarization vector with time. This method wasn’t built upon adiabatic approximation, and it is suitable for more complicated situations, such as systems with relaxation.