The Yangtze River Delta (YRD) is one of the most developed and heavily polluted regions in China. Understanding the origin of fine particles is necessary for policy making on air pollution control in this region. However, the current studies on quantifying the contributions from different emission source regions are limited, especially for winter pollution. In this study, field observations and scenario analysis simulations were performed by using the Weather Research and Forecasting and Community Multi-scale Air Quality (WRF/CMAQ) model to quantify the impacts of local emissions and long-range regional transport in a winter period of 2011. The results indicated that the local contributions were dominant on average in the whole period, especially for slight or medium pollution hours. However, when particle pollution increased, the effects of long-range regional transport significantly increased. The inner-regional effects from Jiangsu and the outer-regional effects from the upwind adjacent province, Shandong, are crucial for the YRD region. The dominant contributor could be local accumulation or long-range regional transport, which usually depends on the weather conditions. This study provides a systematic analysis of the characteristics and origins of winter pollution in the YRD according to combined observations and modeling, which is helpful for understanding regional particle pollution formation and policy making on improving air quality in this region.