Background There are few studies on the diet quality of patients with thyroid cancer, and the relationship between diet quality and thyroid cancer remains uncertain. Objective This study aims to assess the diet quality with the Chinese Health Diet Index (CHDI) and to explore the relationship between diet quality and papillary thyroid carcinoma (PTC). Methods A 1∶1 gender- and age-matched hospital-based case-control study included newly diagnosed PTC patients and matched controls from Shanghai Cancer Hospital and Renji Hospital (East) in Shanghai, China. A structured questionnaire was applied to collect data on general characteristics, history of diseases, dietary intakes, and lifestyles. Food intakes in the past one year were assessed using a validated food frequency questionnaire, from which the CHDI score was calculated. The CHDI, according to the Dietary Guidelines for Chinese Residents, was employed to evaluate the diet quality of the two groups. A multiple conditional logistic regression model was conducted to explore the relationship between diet quality and PTC. Results A total of 350 pairs of cases and controls were recruited. The overall median CHDI score of the cases was lower than that of the controls (67.8 vs. 73.4, P<0.001). The cases had lower median scores of fruits (6.8 vs. 9.5), dairy products (3.6 vs. 5.6), and soybeans (4.6 vs. 5.5) than the controls (P<0.05); the cases had a higher median score of refined grains than the controls (5.0 vs. 4.9), and the percentage of the cases that met diet recommendations for refined grains was higher than the percentage of the controls (65.4% vs. 48.6%) (P<0.05); the cases showed lower median scores of whole grains/beans/tubers, total vegetables, dark vegetables, and fish/shrimps (0.9 vs. 1.4, 3.1 vs. 4.4, 3.6 vs. 5.0, and 3.3 vs. 4.0, respectively), and the percentages of the cases meeting their diet recommendations were lower than the percentages of the controls (6.3% vs. 8.6%, 32.6% vs. 42.0%, 38.6% vs. 50.6%, and 34.0% vs. 40.3%, respectively, P<0.05). The results of multiple conditional logistic regression analysis suggested that qualified and good diet quality were associated with a reduced the risk of PTC (qualified diet quality, OR=0.37, 95%CI: 0.23−0.62; good diet quality, OR=0.19, 95%CI: 0.10−0.36); the statistical significance remained after excluding patients who had a history of benign thyroid conditions (qualified diet quality, OR=0.28, 95%CI: 0.15−0.52; good diet quality, OR=0.20, 95%CI: 0.09−0.43). Conclusion Those with qualified or good diet quality have a lower risk of PTC. PTC patients have insufficient intakes of fruits, dairy, soybeans, whole grains/beans/tubers, vegetables, and fish/shrimps.