The new international accelerator facility FAIR (Facility for Antiproton and Ion Research) is currently under construction in Darmstadt, Germany. The core component of the complex is the heavy ion synchrotron SIS100 which utilises 417 superconducting magnets assembled in 191 cryo-magnetic modules. The SIS100 machine is supplied with two phase helium via 52 local cryogenic components. All cryo-modules are tested at ambient (300 K) and at operating (4.5 K) conditions during comprehensive acceptance tests either at contractor’s or at GSI side. Besides the testing of standalone modules, the SIS100 magnet string will be assembled and tested at GSI. The string will be built-up of a number of the main cryo-magnetic components such as dipole- and quadrupole modules and the most crucial local cryogenic parts, such as a by-pass line piece. A wide spectrum of tests will be performed in order to evaluate the interaction of the main components during cool down, warming up and during powering at operational conditions. The testing program includes i.a. mechanical and electrical integrity tests. Special attention will be brought to the functionality of the ultra-high vacuum system and its performance. Moreover, the quench detection system and the power converter control will be verified on the string. Besides the tests relevant to the SIS100 operation, the string will provide an ideal scenario to develop the processes for the future machine assembly, i.e. sc-circuits interconnection, process pipes welding. In this work the preparation plan of the string test is presented.