Introduction: Functional constipation (FC) is one of the common gastrointestinal disorders that affects people of almost every age. Persistent FC significantly affects quality of life and well-being along with economic burden on patients as well as health care system. Therapeutic efficacy of currently used treatment strategies becomes limited shortly after their discontinuation as constipation occurs again as a result of inappropriate dietary habits. Previous studies have revealed that light vegetarian diet (LVD) can significantly improve both typical and atypical subtypes of major traditional Chinese medicine (TCM) FC syndrome such as gastrointestinal damp-heat syndrome. This protocol aims at exploratorily investigating effectiveness and safety of LVD following a rigorous clinical trial. Methods and design: Total 92 patients in each of the 2 subtypes will be recruited in China-Japan Friendship Hospital for participating in this prospective, placebo-controlled, randomized trial and exploratory study. The patients in each subtype will be randomly divided into 4 groups according to 1:1:1:1 ratio with allocation concealment, which are drug + diet group, drug group, placebo + diet group and placebo group. Patients in the group with diet intervention will be required to strictly follow the LVD. The study will continue for a period of 28 days, including a drug or placebo supervised intervention and a 14th-day telephone follow-up. During the intervention, patients will be required to record a designed diary for controlling the diet quality (DQ) and analyzing the defecation. The study will focus investigation of complete spontaneous bowel movements (CSBM) per week as its primary outcome and constipation-related symptom rating scale (CSS), TCM syndrome scale (TCMSS), 48-hour gastrointestinal transit time (48-hour GITT), high resolution anorectal manometry (HRAM) and fecal flora detection (FFD) will be included in secondary outcomes. Furthermore, the study will also determine safety, DQ and compliance indicators.