A suspended self-balancing self-cruising online water quality monitoring device, an online water quality monitoring method, and an online water quality assessment method are provided. The device includes a suspension cabin main body, a communication and control system, and a suspended carrying platform configured for self-balancing attitude adjustment. Suspension feet configured to drive the suspension cabin main body to implement self-balancing attitude adjustment, cruising, or fixed-point suspension are arranged outside the suspension cabin main body. The communication and control system is configured to plan a W-shaped water-region cruising path and sampling points thereon according to a topography of a water region, control the water sample testing device to test quality of water along the sampling points online, assess water quality of the water region according to an online testing result, and output the water quality of the water region. The device achieves precise positioning and self-cruise monitoring are achieved.