The invention belongs to the technical field of novel photoelectric material deposition preparation, and provides a preparation method of an InN thin film through low-temperature deposition on a self-supporting diamond thick film by the ECR-PEMOCVD, and by means of the preparation method , the InN photoelectric thin film with good electrical properties and heat dissipation performance can be prepared. The preparation method comprises the steps as follows: 1), a self-supporting diamond thick film substrate is ultrasonically cleaned by acetone, ethanol and deionized water in sequence, blow-dried by nitrogen and sent to a reaction chamber; and 2), the ECR-PEMOCVD system is adopted to vacuumize the reaction chamber, the substrate is heated in a range from 20 DEG C to 600 DEG C, then trimethylindium carried by hydrogen and nitrogen are introduced into the reaction chamber, a flow ratio of trimethylindium to nitrogen is (1-2):(100-200), total gas pressure intensity is controlled in a range from 0.8 Pa to 2.0 Pa, electron cyclotron resonance reaction is performed for 30 min-3 h, and the InN photoelectric thin film on the self-supporting diamond thick film substrate is obtained.