Plain river plays an important role in the rapid economic and social development, the health of ecosystem is increasingly concerned. According to the actuality in Yuyao city, 37health diagnosis indexes for the plain river were determined preliminary. They reflected respectively five targets that were the water clear, smooth flow, shore green, beautiful candscape and protection. By using the principal component analysis method the initial indexes were screened , finally 10indicators as the health diagnosis index were deter mined, and through the expert consultation method the weight assignment was giren. On this foundation, the health diagnosis mathematics model for plain riverin Yuyao City has been established, the diagnostic results are divided five grades as health, basic health, sub-health, unhealthy and sick. Taking Longshanpu for example to judge the health condition, the results of diagnosis showed that the comprehensive health index of Longshanpu in 2010is 0.82, namely the health diagnosis result is basic health.