In this paper, 761 events of LFsky-wave sudden phase anomaly (SPA) during solar flares observed at Wuchang Ionospheric Observatory (30°32'40.6"N, 114°21'21.32"E) are analysed. The characteristic of SPAand a quentitative relation between X-ray fluxes and SPAphenomena of LFsky-wave are further investigated. It is pointed out that SPAduring solar flares varies with season and is asymmetric about noon. It is shown that the relation between the SPAand the X-ray logarithmic flux is non-linear. Theoretical results agree with experimental ones.