Oil shale in-situ retorting is a reasonable development technology. However, the ground water may flow into fractures in oil shale layer that impact the process of oil shale in-situ retorting. This paper introduced oil shale in-situ fracturing-nitrogen injection exploitation and a method of dynamic pressure balance between the ground water and high pressure nitrogen to keep the oil shale layer without ground water in the process of oil shale in-situ fracturing-nitrogen injection exploitation. Theoretical basis of dynamic pressure balance between ground water and nitrogen was established through analyzing pressure relationship between ground water and nitrogen in the fractures and field experiment was conducted according to the method. The field experiment results showed that nitrogen pressure maintained high level in the fractures during the stage of building pressure balance of nitrogen and ground water and pushed ground water out of the oil shale layer. Then, nitrogen pressure in the fractures reduced and maintained stable because part of nitrogen in the fractures flowed out from the production well and flow conductivity of fractures enhanced. After the balance between the ground water and high pressure nitrogen was established, water yield of production well reduced more than 85%. It explained that the balance has function of sealing up.