Abstract: Soil nutrients are important for the growing of Larix principis-rupprechtii. Soil content of available K for 5a, 10a, 20a, 30a, 40a Larix principis-rupprechtii was measured and analyzed of May to October to find out seasonal dynamics in soil content of available K for different ages of Larix principis-rupprechtii in the northern foot of the Qinling. The results showed that in different months soil content of available K for different ages of Larix principis-rupprechtii was declined from the soil depth of 0-20cm to 20-40cm. On the soil depth of 0-20cm, soil content of available K of 30a and 40a Larix principis-rupprechtii was higher than other ages,. Soil content of available K for different ages of Larix principis-rupprechtii in different months is significantly different（P0.01）. The above results suggests that soil content of available K for different ages of Larix principis-rupprechtii without fertilization reach the middle and high levels，soil content of available K of 30a and 40a Larix principis-rupprechtii was higher than other ages，. On the soil depth of 0-20cm, soil content of available K of different ages Larix principis-rupprechtii has a parabolic variation which increases firstly and then decreases. Soil content of available K of Larix principis-rupprechtii plantation in the northern foot of the Qinling is abundant, the supply of available potassium is adequate.