GSI plans to build a test string in the Series Test Facility (STF) for SIS100 at GSI. The SIS100 cryomagnetic modules and local cryocomponents will be assembled and intensively tested. The purpose of the string test is to confirm technical systems of cryogenic, vacuum, interlock and quench protection, and powering under normal and extreme conditions. Acquirement of knowledge through experiences in test string construction, commissioning, and testing is crucial for the SIS100 construction and operation. We present the planning and preparation status of the string test.