NOVELTY - Mixed organic fertilizer preparation involves mixing sylvite and biomass material in oxygen-deficient or oxygen-free condition and performing thermal decomposition for 1-3 seconds at 500-550 degrees C. The thermal decomposition gas is collected and condensed to room temperature to obtain a liquid product, solid product and incondensable gas. Nitrogenous substance is added to the obtained liquid product in oxygen-deficient or oxygen-free condition and stirred uniformly at increment of 1-2 degrees C/minute, where temperature is raised up to 120 degrees C.    USE - Method for preparing mixed organic fertilizer (claimed).    ADVANTAGE - The mixed organic fertilizer preparation method provides mixed organic fertilizer having good slow-release effect of nitrogen fertilizer in cost-effective manner.    DETAILED DESCRIPTION - Mixed organic fertilizer preparation involves mixing sylvite and biomass material in oxygen-deficient or oxygen-free condition and performing thermal decomposition for 1-3 seconds at 500-550 degrees C. The thermal decomposition gas is collected and condensed to room temperature to obtain a liquid product, solid product and incondensable gas. Nitrogenous substance is added to the obtained liquid product in oxygen-deficient or oxygen-free condition and stirred uniformly at increment of 1-2 degrees C/minute to obtain a mixture, where temperature is raised up to 120 degrees C. The obtained solid product is added into the mixture and heated continuously up to 130 degrees C by maintaining temperature increment of 1-2 degrees C/minute. The obtained mixture is oxidized for 3-6 minutes in presence of air by heating to 140 degrees C and preserved for 5-10 minutes. The oxidized mixture is cooled and solidified to obtain the mixed organic fertilizer composed of potassic fertilizer and slow-release nitrogen fertilizer. An INDEPENDENT CLAIM is included for a thermolysis device, which comprises a fluidized bed thermolysis reactor, a cyclone separator and a condenser, where inlet of the fluidized bed thermolysis reactor is connected with charging device and outlet of the fluidized bed thermolysis reactor is connected with the inlet of the cyclone separator, and gas outlet of the cyclone separator is connected with the condenser.