The invention discloses a simple measuring device and a simple measuring method for an underwater topography under a complex water flow condition, and belongs to the technical field of design and manufacture of technological equipment for water conservancy and hydropower construction projects. The simple measuring device for the underwater topography under the complex water flow condition is easy to operate and accurate in measurement result. The simple measuring method is an operation method which is used for measuring the underwater topography by using the simple measuring device. The simple measuring device comprises a pendant, a driving mechanism, a measuring mechanism and a supporting frame. The simple measuring method comprises the following steps of throwing the pendant in water flow in which the underwater topography requires to be measured; pulling the pendant by using the driving mechanism along the extension direction in which the underwater topography requires to be measured; stopping and stabilizing the driving mechanism at various specific positions at which the underwater topography requires to be measured respectively according to the design requirement; measuring the length of a portion, which is exposed out of the water surface, of the measuring mechanism by using an external measuring scale; and performing simple calculation and drawing a calculation result graph to finish measurement operation of the underwater topography.