The spatial distribution of the ECR plasma density has been measured by using an eccentric Langmuir probe. The result indicates that the plasma density is very uniform in the axis Z=50 cm and radial Φ=12 cm. Effect of the radial uniformity of plasma density on the uniformity of deposition rate and thin film thickness is analyzed. The repeatability to prepare silicon nitride thin film of a specified thickness is discussed. The relation of the deposition process with the deposition rate of silicon nitride thin film is investigated and the dependence of the practical application on process parameters has been obtained for the deposition thin film with ECR-PECVD technology.