The magnetograms of the longitudinal field in 18 actively regions for April-December, 1988 are analysed in the paper (Huairou Station, Beijing Observatory provieded the magneto-grams). Some magnetic precursors of SWFflares (The flare causing strong Short Wave Fade-out is called as SWF flare) are distingushed from the magnetograms of the longitudinal field. They are:1. Emergence of the magnetic flux. Amagnetic flux emerges in a magnetic area with the opposite polarity (Bl
≥320G). The intensity of emerging magnetic flux is larger or equal to 160G.2. Incorporation of the magnetic fluxes with similar polarity. Among the magnetic fluxes of the similar polarity participating in the incorporation, the strongest magnetic intensity is equal to or larger than 320G, tthe weakest is equal to or larger than 80G. The maximum magnetic gradient near the incorporating area after the incorporation is equal to or larger than 0.15G/km.3. Invasion of a magnetic flux with opposite polarity. Amagnetic flux (Bl
≥320G) invades into an area of the magnetic flux with opposite polarity (Bl
≥320G) and is continuouisly encircled by the magnetic flux with opposite polarity to 70% of all or more.4. Asharp increase in the magnetic intensity. The growth rate of magnetic intensity in an area of the magnetic flux is 960 G/day or more.There is a close relation between the SWF flares and the magnetic precursors.