Abstract: Plant leaf area calculation is the key content of plant field survey, it is the k in the traditional formula of plant leaf area s=kab, that cause difficulty to apply it for its varying with the difference species. In order to facilitate the correct choice of k values, nine kinds of representative plant leaves forms were selected as samples to calculate the k. the procedure of the k calculating was as, six indexes of the plant leaf was calculated with Image Pro Plus 6.0, those were the area of the plants leafs, ratio of the leaf area with the bounding rectangle of which, the maximum diameter, the minimum diameter, length and width. According traditional classification of plant leaves, the plants leafs was divided into four kinds by the ratio of the maximum diameter and the minimum diameter, these were lanceolate, long oval, ovoid and round, respectively, the values were 1/3, 1/2, 2/3 and 5/6. The test proved that, the average error of k was 4%; however, it was 20% when the k taken 2/3. When k value has been determined with the leaf forms, it is easier to choose the k value when working in fields. This will prompt the working efficiency due to its simplicity.