An over-forecasetd extratropical cyclone during 12-16 February 2015 in East Asia was studied by Date assimilation and Observation impact. The development of extratropical cyclone is different from some Numerical Weather Prediction Centers'simulations because of non-exist explosive development. By means of WRF and Date Assimilation model, 3DVAR experiment assimilated NCEP ADP Global Upper Air and Surface Weather Observations and reduced forecast error. With the decrease of cyclonic circulation and atmospheric north-south temperature difference which reduces baroclinity, the moment of intensification is delayed and intense is weaker than NWPC's simulation. Meanwhile, 12-hours observation impact was evaluated by WRF adjoint model WRFPLUS and WRFDA-FSO. For each observation type, the total contribution of reducing forecast error was greatest for SOUND, SYNOP, GEOAMV. Sensitive experiment which assimilated three above observations provides more ideal prediction and improves simulation result.