The invention discloses a data processing system used for choosing an off-filed mode before navigation, wherein the system can be used for determining automatically a best off-field mode before a plane takes off and the system has advantages of high working efficiency and strong security. The system comprises the following parts: a flight plan subsystem, which is used for receiving information ofa fight plan and sending a request of the flight plan; an airport control information subsystem, which is used for dealing with and parsing information of air traffic control of the present airfield;an off-field mode choosing subsystem, which is connected with the flight plan subsystem and the airport control information subsystem and makes calculation on the off-field mode according to a choosing logic and a database; and a choosing information verification subsystem, which is used for verifying a choosing result according to historical information. In addition, the invention also provides a method by employing the system.