The invention discloses high-rigidity, high-strength and pitting-resistant ultra-fine grain stainless steel and a preparing method thereof. The stainless steel is prepared from, by mass, 64% to 70% of Fe, 18% to 20% of Cr, 7% to 9% of Ni, 1% to 3% of Mo and the balance impurities. According to the stainless steel and the preparing method, the mechanical alloying preparing technology is mainly used, and the three kinds of elements including Cr, Ni and Mo are solidly fused in Fe to form a single BCC structure. In the hot pressing sintering process, the structure is converted into austenite, and a single austenite structure is formed. Then heat treatment is carried out on the austenite structure, so that the elements of the austenite are evenly distributed, stress left after mechanical alloying is eliminated, and finally the stainless steel with the rigidity higher than 400 HV, the tensile strength of about 900 MPa and the corrosion resistance much higher than 316 L is obtained.