As a typical form of the Serverless architecture, the function as a service (FaaS) architecture abstracts the business into fine-grained functions, and provides automatic operation and maintenance functionality such as auto-scaling, which can greatly reduce the operation and maintenance costs. Some of the high concurrent, high available, and high flexible services (such as payment, red packet, etc.) in many online service systems have been migrated to the FaaS platform, but a large number of traditional monolithic applications still find it difficult to take advantage of the FaaS architecture. In order to solve this problem, a FaaS migration approach for monolithic applications based on dynamic and static analysis is proposed in this study. This approach identifies and strips the implementation code and dependencies for the specified monolithic application API by combining dynamic and static analysis, and then completes the code refactoring according to the function template. Aiming at the cold-start problem of functions in high concurrency scenario, this approach uses the master-slave multithreaded Reactor model based on IO multiplexing to optimize the function template and improve the concurrency processing capability of a single function instance. Based on this approach, Codext, a prototype tool for Java language, is implemented and experimental verification is carried out on OpenFaaS, an open source Serverless platform, for four open source monolithic applications.