Abstract: Change of structure in specimen cutting from casting slab of non-oriented silicon steel (/% ： 0. 001 8C, 0.76Si, 0.24Mn, 0.074P, 0.003S, 0.001 Al, 0.005 60,0.001 IN) cooling from 1 400 ℃：to 600 ℃. with 5 ~40 ℃/s rate has been observed at real-time by using a high temperature confocal microscope, and after observation the inclusions in specimen are extracted and analyzed by non-aqueous solution electrolytic method. Results show that steel cooling to the two precipitation period i. e. beginning precipitated at 987 ~ 1 020 ℃ (higher temperature) and 830 ~ 636℃(lower temperature) and finishing precipitated at 952 ~918 °C and 804 ~ 636 ℃. there are new phases respectively precipitated； in first (higher temperature) period with increasing cooling rate the beginning and finishing temperature of new phases all increase but the precipitated temperature period decreases, at second (lower temperature) period with increasing cooling rate the beginning and finishing precipitated temperature of new phases all decrease but the precipitated temperature period increases, beside at lower cooling rate the higher precipitated temperature period is favorable to sufficient precipitation of new phases, and at higher cooling rate the lower precipitated temperature period is favorable to sufficient precipitation of new phases ； with increasing cooling rate the inhibition effect of inclusions on grain growth gradually decreases, the average grain size of steel first decreases and comes down to minimum value with cooling rate 10 ℃/s, then monotonically and quickly increases. Therefore the cooling rate less than 5 ℃/s or more than 20 ℃/s should be selected to avoid forming 0.21 ~ 0.50 μm MnS and AIN, as well as Cu_xS inclusions precipitated in cooling later stage to get better magnetic performance.