The invention discloses a kind of straw composite organic fertilizer and a preparation method thereof. The straw composite organic fertilizer contains the following components: 200-300 parts by weight of straws, 20-30 parts by weight of soybean curb residue, 10-20 parts by weight of vinasse, 5-8 parts by weight of phosphorite powder, 6-10 parts by weight of humic acid, 2-4 parts by weight of trace elements, 4-6 parts by weight of a composite microbiological inoculant, 1-3 parts by weight of mint powder, 0.8-1.5 parts by weight of lycorine, 3-5 parts by weight of pericarpium granati powder, 1-2 parts by weight of sodium potassium tartrate and 4-7 parts by weight of activated carbon. The straw composite organic fertilizer disclosed by the invention has the advantages that the straw is adopted as a main raw material, the sodium potassium tartrate and activated carbon are added, heavy metal elements in soil can be removed, and the aim of purifying the soil is achieved, so that healthy growth of crops is guaranteed; and the mint powder, lycorine and pericarpium granati powder are added, so that the fertilizer has functions of expelling parasites and avoiding harm, the soil has disease-and-insect-resistant capability, and yield of the crops is increased.