The reasonable water pressure upon lining determination is the key to structure design of subsea tunnel. The analytic method calculation formula of outside pressure upon lining of circular subsea tunnel was derived with complex function and groundwater dynamic theory. It was verified that the calculation formula is high precision and extensive applicability by lots of contrast. Results of model tests showed that under the condition of fully closed, the pressure on lining could not be reduced; the water pressure would significantly reduce if the water of the tunnel was drained, and it would be presented linear decreasing with the increase of the water discharge. Lining external water pressure formula under limited drainage was established, and it was also verified by field tests. The research results were applied in the first subsea tunnel in China, Xiang'an Subsea Tunnel, which is built in Xiamen. And it provided key technique parameter to determination of support structure design and programme of blocking water by grouting, following the principle plugging dominantly and limited drainage. The field application obtained good results. Research results would have certain reference significance to the following subsea tunnel.