The possibility of preserving the proton polarisation in the Nuclotron using weak and strong longitudinal magnetic fields is discussed. It is sufficient to use a weak solenoid with a field integral up to 50 mT⋅m to preserve the proton polarisation in the momentum range up to 3.5 GeV/c. In this momentum range, there are no intrinsic resonances, and to preserve the proton polarisation at the crossing of integer spin resonances, the correcting dipoles of the Nuclotron are used in addition to the weak solenoid. The insertion of a solenoidal field with an integral up to 24 T⋅m will make it possible to preserve the proton polarisation in the whole momentum range of the Nuclotron. It becomes possible to perform first-priority experiments with polarised beams at the Nuclotron on both internal and external targets. The obtained results will allow to use the Nuclotron as an injector of polarised protons into the NICA Collider.