We aimed to explore the correlation between plant-based diet indices and abdominal obesity with depression and anxiety among older Chinese adults. This study used a cross-sectional design using data from the 2018 Chinese Longitudinal Healthy Longevity Survey (CLHLS). We used a simplified food frequency questionnaire to evaluate the overall plant-based diet index (PDI), the healthful plant-based diet index (hPDI), and the unhealthful plant-based diet index (uPDI) separately, based on the potential health effects of the foods. Waist circumference (WC) was used to define abdominal obesity. The 10-item Center for Epidemiologic Studies Depression Scale (CES-D-10) and the 7-item Generalized Anxiety Disorder Scale (GAD-7) were applied to estimate depression symptoms and anxiety symptoms, respectively. Multi-adjusted binary logistic regression models were conducted to explore the effects of the three plant-based diet indices, abdominal obesity status, and their interaction on depression and anxiety. We enrolled a total of 11,623 participants aged 83.21 ± 10.98 years, of which 3140 (27.0%) participants had depression and 1361 (11.7%) had anxiety. The trend in the prevalence of depression/anxiety across increasing quartiles of the plant-based diet indices was statistically significant after controlling for potential confounders (p-trend < 0.05). Abdominal obesity was related to a lower prevalence of depression (OR = 0.86, 95% CI: 0.77−0.95) and anxiety (OR = 0.79, 95% CI: 0.69−0.90) compared with non-abdominal obesity. The protective effects of the PDI and hPDI against depression (OR = 0.52, 95% CI: 0.41−0.64; OR = 0.59, 95% CI: 0.48−0.73, respectively) and anxiety (OR = 0.75, 95% CI: 0.57−1.00; OR = 0.52, 95% CI: 0.39−0.70, respectively) were more pronounced in non-abdominally obese participants. The harmful effects of the uPDI against depression (OR = 1.78, 95% CI: 1.42−2.23) and anxiety (OR = 1.56, 95% CI: 1.16−2.10) were more pronounced in non-abdominally obese participants. In addition, a significant interaction between the plant-based diet indices and abdominal obesity was observed in terms of causing the prevalence of depression and anxiety. Consuming more of a healthful plant-based diet and less of an animal-based diet is related to a lower prevalence of depression and anxiety. A healthful plant-based diet plays a vital role in non-abdominally obese individuals.