Apple fruit sunburn is a common disorder and one of the distinguishable symptoms is that browning color appears on the exposed side of a fruit. It has been taken for granted that PPO is absolutely involved in the browning process of plant tissues or organs under stressed conditions. The present experiment was conducted to examine the effect of high temperature and excessive light stresses on PPO activity in fruit peel under both laboratory and field conditions, so that the relationship could be disclosed between the occurrence of fruit browning and PPO activity in peel tissues stressed by high temperature and excessive solar radiation. The results indicated that a close relationship was found between the stressed extent of both high temperature and excessive light, and PPO activity in apple peel. As far as different exposures of a tree canopy were concerned, the highest PPO activity existed in exposed fruits on SW exposure where they were frequently stressed most severely. Within a certain range, PPO activity in fruit peel increased gradually with a rise of temperature and light intensity and an abrupt temperature increasing could result in a dramatic growth in PPO activity. Under the same high-temperature stressed conditions, a higher relative humidity was favorable to inhabiting PPO activity in fruit peel, thus reducing the risk of heat-related damage. The results consistently confirmed that the browning process of sunburn fruit was associated directly with the significant increase in PPO activity in fruit peel.