Entamoeba histolytica and Giardia lamblia are two types of parasites that can cause intestinal infections in humans. Patients can be transmitted through the ingestion of food and drinking water contaminated with cysts, and present symptoms such as abdominal pain and diarrhea after infection. If Entamoeba histolytica infection is not actively and effectively treated, patients may also present with clinical manifestations such as purulent and bloody stools and colonic ulcers. This study reports a diarrhea case with concurrent infection of Entamoeba histolytica and Giardia lamblia. During the diagnostic process, the parasites were rapidly detected using the direct saline smear method. Subsequently, iodine staining and iron hematoxylin staining techniques were employed to meticulously observe the internal structures and morphological features of the parasites under a microscope, enabling successful identification of the parasite species. The diagnostic process in this case suggests that laboratory personnel should further enhance their ability to recognize the morphological characteristics of cysts and trophozoites of Entamoeba histolytica and Giardia lamblia, providing an effective basis for the clinical differential diagnosis between Entamoeba histolytica infection and ulcerative colitis.