Abstract: This paper made a numerical simulation of magnetic reconnection of highly stressed force-free field triggered by an instantaneously forned current sheet,using the multistep implicit scheme.Magnetic reconnection takes place in the non-neutral current sheet between the two fields at first,and then extends toward the for ce-free field region.During the magnetic re-connection,a hightemperature loop structure forms in the force-free field.The mass in the loop flows to the photosphere whereas the mass in the newly emerging field region on the inner side of the loop moves upward.This result agrees with the observation that solar flares occur in the red shift side of the inversion line of H chromospheric Dopplergiram.Magnetic reconnection concentrates mainly in the highly stressed force-free field region on the exterior side of the car-rent sheet.The loop has a higher temperature at its apex and keeps nearly stationary.As the magnetic reconnection proceeds,the gradient of sheared magnetic field component decreases and force-free field factor becomes smaller.The results of this paper may be applied to elucidate single-loop flares.