The invention provides a control method of a washing and drying all-in-one machine, a control device for the washing and drying all-in-one machine, and the washing and drying all-in-one machine. The control method comprises the following steps: acquiring a first weight of a load in a washing drum of the washing and drying all-in-one machine before water enters the washing and drying all-in-one machine, and acquiring a spinning speed; calculating first drying time according to the first weight, the spinning speed and the drying efficiency of the washing and drying all-in-one machine, and carrying out preliminary drying. By adopting the technical scheme provided by invention, the water content of clothing can be accurately known, and the drying time can be initially determined, so that the problem that a motor can not discriminate the weighing difference before and after washing of the micro load is solved, and the problem that when a user puts wet clothes into the machine, the drying program is not performed due to a small weighing difference value before and after washing is also solved; therefore, the situation of over-baking or not baking to dryness is avoided, and the drying effect is effectively improved. Compared with related washing and drying all-in-one machines, the washing and drying all-in-one machine provided by the invention is more intelligent, water-saving and power-saving.