Abstract:The photosynthesis rate, transpiration rate, chlorophyll content and treegrowth are all found to be decreased remarkably by 2-3 years‘ repeated over-pruning (only two branch whorl reserved) or over-pruning plus defoliation in Pinus tabulaeformis stand which has been closed to human activities for eightyears. As a result, the decreased content of soluble carbohydrate and starch,and the increased contents of fat and protein make the total quantity of heatin needles as insect food increased. This benifits the weight growth rate of Dendrotimus larvae, and weakens tree resistance to insect. Therefore not onlythe photosynthesis rate and tree vigour, but also its resistance to insect couldbe increased through reasonabl pruning. Experiments show that most current needles of Pinus tabulaeformis are onthe fourth branch whorl, and they are extremely important to accumulatingphotosynthetic products. Based on the experiment result, the standard of reasonable pruning of Pinus tabulaeformis is discussed.