The invention relates to a nanowire self-reinforced porous silicon nitride ceramic and a preparation method thereof. The technical scheme is as follows: the preparation method comprises the following steps: using 70-80 wt% of silicon powder, 5-10 wt% of catalyst and 10-20 wt% of nitrogen source as raw materials, adding deionized water which accounts for 20-30 wt% of the raw materials, and stirring to obtain a ceramic slurry; adding foam prepared by adding a foaming agent (accounting for 10-20 wt% of the raw materials) into the ceramic slurry, and continuously stirring for 30-60 minutes to obtain a ceramic foam slurry; pouring the ceramic foam slurry into a mold, standing in a nitrogen environment, drying, and demolding to obtain a ceramic body; in a nitrogen atmosphere, heating the ceramic body to 1100-1150 DEG C, and keeping the temperature; heating to 1200-1600 DEG C, and keeping the temperature; and naturally cooling to obtain the nanowire self-reinforced porous silicon nitride ceramic. The method has the advantages of simple technique, low cost, high utilization ratio of raw materials and controllable process. The prepared product has the advantages of uniform pore size, uniform pore size distribution and high mechanical strength.