Citrus trees (Citrus Linn) are important economic craps for south China. They must grow in warm and humid cimate. Freeze injury in winter is the main weather calamity of the northern fringe of citrus trees growing region of China. It affects the normal growth, development, output and quality of citrus.The method for ecological division of citrus trees in a county is discussed. Citrus trees are one of the typical frost tender plants in subtropical zone. Jiangshan County is a new citrus growing area of Zhejiang Province. It is situated at the northern fringe of citrus growing region. This county can be divided into four ecological areas of cultivating mandarin (Citurus reticulata) according to the major ecological factors (topography, climate, and soil), namely: near suitabe (Ⅰ), poor suitable (Ⅱ), possible growing (Ⅲ), and non-growing (Ⅳ), and nine sub-areas according to the condition of citrus freeze injury in winter and citrus dry damage in summer. Ecological conditions of area Ⅰ are basically suitable for mandarin growing.