The intriguing problem during polymer crystallisation is to understand the crystal formation and transition, in particular the initiation in a liquid melt close to solid crystals and one solid crystal close to another. Polymorphic isotactic polybutene-1 (iPB-1) usually crystallises into kinetically favoured metastable form II from melt and then spontaneously transits to thermodynamically stable form I. Keeping in mind that polymer crystallisation is controlled by temperature-dependent prior chain conformation formation, we first successfully conserved form-I-favoured 3/1 helix conformation at low temperatures and determined its decomposition temperature as Td,3/1 = 132 °C. The detailed temperature-dependent helix conformation evolution of iPB-1 during the cooling/heating processes was outlined and the subsequent easy direct form I crystallisation via necessary diffusion and alignment of the preserved 3/1 helix conformation was deduced. The results would be helpful to understand the helix conformation roles in polymer crystallisation, polymorph selection and crystal transition.