Downhole constant pressure valve for oil tube

The invention discloses a downhole constant pressure valve for an oil tube. The downhole constant pressure valve for the oil tube comprises a valve body and a spool. The spool is disposed in the valve body. A gap between the spool and the middle of the valve body is sealed by a spring. The spring is located on a spring holder connected to the lower portion of the valve body. The upper portion of the valve body is connected with a bonnet through a shear pin. The lower portion of the valve body is provided with a water hole for drilling fluid circulation. The upper portion of the bonnet is connected with the oil tube, an anti-fall cap holding the valve body from falling is connected to the lower portion of the bonnet, and a gas production mesh sealed by the valve body which does not fall yet is arranged in the middle of the bonnet. The downhole constant pressure valve in the mechanical structure is capable of effectively and temporarily plug a running oil string when inside the running oil string, so that leaka