Background: Exploring emerging dietary patterns, such as plant-based diets (PBD), often requires considerable effort to rebuild new systems or adapt existing food classification frameworks, presenting a substantial challenge for dietary research. Current systems were not originally designed for this purpose and vary in standardization and interoperability, complicating cross-study comparisons. This study aimed to adopt the harmonized, food description-incorporated, food classification system (HFDFC system) to develop a plant-based diet food classification system (PBDFC system), and to evaluate dietary intake and nutritional status among adults in Taiwan. Methods: A repeated cross-sectional design was applied using 24 h dietary recall data from the Nutrition and Health Survey in Taiwan (2013–2016 and 2017–2020), accessed via the national food consumption database. Adults aged 20–70 years were included. Data were processed through the HFDFC system to generate the PBDFC system. For each participant, the Plant-Based Diet Index (PDI), Body Mass Index (BMI), and Nutrient-Rich Food Index (NRF) were calculated and analyzed by age group. Results: Adults aged 46–70 had significantly higher O-PDI and H-PDI scores, lower Lh-PDI scores (all p < 0.0001), and higher NRF values. Despite higher average BMI, those in the highest H-PDI tertile had significantly lower BMI (p < 0.02). Conclusions: The HFDFC-based PBDFC system offers a flexible, scalable framework for plant-based diet classification and supports future cross-national research.