To explore the effect of magnetic field on the explosion characteristics of premixed gas and its mechanism， taking ethylene as an example， the influence law of magnetic field on the explosion characteristics of premixed ethylene/air was experimentally studied. The chain reaction process of ethylene explosion was numerically simulated. The influence of magnetic field force on key free radicals was theoretically analyzed. The transient pressure of ethylene explosion was measured by pressure sensors and the flame propagation velocities were measured by explosion velocity apparatus. Results show that under the maximum magnetic field intensity of 3300 Gs， the maximum explosion pressure of 6.5% ethylene decreased by 18.18% and the explosion pressure rise rate reduced by 17.33%. Along the flame propagation direction， the magnetic field firstly promoted and then suppressed the flame propagation speed of ethylene explosion， in which the suppression effect was greater than the promotion effect. The ethylene explosion was simulated by Chemkin-pro software， and the key free radicals in ethylene explosion were obtained. Different types of free radicals have different intensities of magnetization. Force of free radical under magnetic field is proportional to magnetization of free radical. The magnetic field force has a greater impact on free radicals with high magnetization， and no impact on anti-magnetic substances. Different types of free radicals appear stratification phenomenon under magnetic field. The collisions between different types of free radicals were reduced， and the elementary reaction rate was decreased， which suppressed the ethylene explosion.