The invention provides a defoaming device for engine oil inside a crank case of an engine. The defoaming device comprises a structure body; the structure body is located below a crank; multiple first open slots, multiple second open slots, multiple third open slots and multiple fourth open slots are arranged and formed in the structure body; the first open slots are located in the front end of the structure body and correspond to the position of a journal of a crank connecting rod; the second open slots are located in the front side of the middle of the structure body and correspond to the position of the journal of the crank connecting rod; the third open slots are located in the back side of the middle of the structure body and correspond to the position of a balancing weight of the crank; the fourth open slots are located in the back end of the structure body and correspond to the position of the journal of the crank connecting rod; and flow guide plates are arranged on the four kinds of open slots. The defoaming device has the beneficial effects as follows: open slot structures at different angles are formed in different positions of the structure body according to the rotation direction of the crank of the engine, air bubbles produced by air flows produced due to movement of the crank are avoided, the flow direction of the engine oil falling back freely is guided, and splashing is prevented.