Abstract: The health assessment of river ecosystems can reflect the quality and stability of river ecosystems. The structure and distribution of periphytic algae communities can indicate long-term changes in the aquatic ecological process, making them highly significant indicators for river water environment health. In summer (June) and fall (September) of 2023, 30 sampling sites were set up in the mainstream of the Heilongjiang River, to investigate the living periphytic algae and water quality. The Water Quality Index (WQI) and periphytic algae index of biological integrity (P-IBI) were used to evaluate the water quality and ecological health of the Heilongjiang River. The results showed that: (1) There were 7 phyla, 95 genera, and 234 species of periphytic algae in the Heilongjiang River, dominated by diatom-cyanobacteria-green periphytic algae. The dominant species in summer were diatoms, while diatoms and cyanobacteria alternately held the advantage in fall. (2) Using Principal Component Analysis (PCA) to select the main environmental factors including COD_Mn, NH₃-N, TN and TP, and combining WQI and Shannon-Wiener diversity index, two core parameters in summer and autumn were selected from 17 candidate biological parameters based on the boxplot discrimination ability (assignment≥2), the health levels were categorized using the 25th percentile as a reference point, and the P-IBI for the Heilongjiang River was built up. (3) The evaluation results showed that both WQI and P-IBI at the referencing points were in "good" condition in both summer and autumn, and the results were highly consistent. At the damaged points, P-IBI was in "good" condition in summer but "poor" in autumn, and WQI was in a "general" state in both summer and autumn. The environmental factors that mainly affected P-IBI of the Heilongjiang River in summer and fall were conductivity and water temperature, respectively, and COD_Mn and COD_Cr reached Class Ⅳ category of Environmental Quality Standards for Surface Water ( GB 3838-2002), which should be closely monitored.