Based on conventional ground observation data of weather stations and pollutant mass concentration observation data from environment monitoring stations in Liaoning Province, ECMWF reanalysis data with 0.5°×0.5° spatial resolution, a heavy pollution process caused by straw burning from 7 to 10 November 2015 in Liaoning Province, the differences and its causes of pollution among different cities were analyzed. The results are as follows: (1) CO was main pollutant during the heavy pollution process. The mass concentration of PM2.5 appeared two peak stages, and it was well corresponded to CO and NO2 mass concentrations. The visibility was mainly affected by PM2.5 concentration and relative humidity. (2) The effect of previous weak precipitation more than 1.0 mm on wet deposition of pollutants was obvious in Yingkou and Panjin, while the precipitation in other cities was smaller, and the high humidity environment was conducive to moisture absorption and growth of pollutants. (3) The horizontal wind speed from ground to 700 hPa during the heavy pollution was close to 4 m·s-1, the atmospheric stratification was stable, and the temperature inversion was obvious, which was beneficial to inhibit the vertical diffusion of pollutants. (4) The warm layer around 0 ℃ between 850 hPa and 900 hPa over cities except for Jinzhou maintained for a long time, which might provide favorable environment for the melting, collision and growing of pollutant particles. (5) There were many fire points in Jilin and Heilongjiang Province during the pollution process. Therefore, the main sources of pollution in central Liaoning Province came from the wide straw burning upwind, so it was necessary to strengthen the control of local and external pollution sources under the favorable weather conditions to the pollutants accumulation.