The energy dissipation mechanism in blazar jet is unknown. Blazar flares could provide insights into this problem. Here we report statistical results of XMM-Newton observations of X-ray flares in Mrk 421. We analyze all public XMM-Newton X-ray observations for Mrk 421, and construct the light curves. Through fitting the light curves, we obtain the flare-profile parameters, such as peak flux (F-p) and flaring time duration (T-fl). It is found that the distributions of F-p and T-fl both obey a power-law form, with the same index of alpha(F) = alpha(T) approximate to 1. The statistical properties are consistent with the predictions for a self-organized criticality system with energy dissipation in one-dimensional space. This is similar to a solar flare, but with different space dimensions of the energy dissipation domain. This suggests that the X-ray flaers of Mrk 421 might be driven by a magnetic reconnection mechanism. Moreover, in the analysis, we find that variability on a timescale of similar to 1000 s appears frequently. This rapid variability indicates a magnetic field of >= 2.1 delta(-1/3)(D) G (delta(D) is the Doppler factor) in the emission region.