The invention discloses an in-situ collecting device for hay entering into a lake and a method for detecting the amount of the hay entering into the lake. The device comprises a fence net device set on a shore of the lake, a tumbleweed collecting pool and a pulling device. Three sides of the fence net device are closed, one side is opened, the tumbleweed collecting pool is located in a space formed by the fence net, and the tumbleweed collecting pool is filled with water. The pulling device includes at least two pulling slide rails, pulling rods and plowshares, the pulling slide rails are arranged above the tumbleweed collecting pool in a front and back direction, the pulling rods are fixedly equipped with sliders which are corresponding to the number and positions of the pulling sliding rails, the plowshares are fixed to lower ends of the pulling rods, and bottoms of the plowshares are attached to a bottom of the tumbleweed collecting pool. When tumbleweed is blown into the lake by wind, the tumbleweed enters the fence net device from the opening of the fence net device, the pulling rods can slide on the pulling slide rails with the sliders, and the plowshares drive wet tumbleweedin the tumbleweed collecting pool to one side of the tumbleweed collecting pool. According to the invention, the amount of the tumbleweed entering into the lake can be accurately detected.