Retinopathy of Prematurity (ROP) mainly occurs in premature infants with low birth weight, and it is the leading cause of childhood blindness. Early and accurate ROP diagnosis is imperative for appropriate treatment. However, less research concentrates on early-stage ROP diagnosis based on limited-labeled images in an imbalanced dataset. To address the dilemma, this study proposed a novel self-supervised network, MOCO-MIM, for early ROP grading. The proposed classification network was evaluated on a total of 553 labeled fundus images from 89 preterm infants. The trained network achieved a test accuracy of 98.29% and an AUC score of 97.6% for three stages of grading. The adopted method is verified that the proposed method can be detected early stages of ROP more efficiently and grade the severity more accurately based on limited-labeled fundus images, which is superior to the existing state-of-the-art methods.