The invention discloses a monitoring method for bridge deformation in shield construction. According to the method, in the shield construction process, bridge deformation monitoring data is obtained by monitoring a railway viaduct, then the data is transmitted and logged for obtaining a database, monitoring analysis is performed according to data in the database, and finally determination is performed. The invention also discloses a system employed by the above method. The system comprises a fixed facility system and an automatic monitoring system; the fixed facility system is used for providing equipment for the automatic monitoring system for monitoring; and the automatic monitoring system is used to performing monitoring by using the fixed facility system for obtaining the bridge deformation monitoring data. The method is capable of realizing continuous dynamic precise monitoring on bridge settlement and displacement statuses when a shield machine passes underneath the railway viaduct, provides control index of the railway viaduct, and is capable of quantitatively evaluating the safety of the railway viaduct.