The invention discloses a steam constant temperature control device of a urea melting system for ureabased compound fertilizer production, which is characterized by comprising a urea melting slot as well as a stirring pump, a urine pump, a digital display temperature control instrument and a steam electric adjusting valve which are all arranged on the urea melting slot, wherein the digital display temperature control instrument is connected with the steam electric adjusting valve. The steam constant temperature control device of a urea melting system for ureabased compound fertilizer production has the following advantages that the steam electric adjusting valve and the digital display temperature control instrument are adopted, therefore, the valve can be automatically closed when the temperature is higher than 130 DEG C and opened when the temperature is lower than 122 DEG C so that the temperature of urine is kept between 122 DEG C and 130 DEG C constantly and can not rise or fall suddenly, the urine cannot form biuret because the temperature raises to 132.7 DEG C or form crystals to block the urine pump because the temperature is lowered, and the product quality of ureabased compound fertilizer and the smooth production of the ureabased compound fertilizer can be ensured.