The invention relates to a Montgomery modular multiplication method and a circuit thereof. In the method, a modular length k=sw, wherein w is a word size processed each time by an algorithm, and s is the frequency required by the circulation of the algorithm for; input parameters include a, b and a modulus n; r is a memory, a high k place is used for accommodating the input parameter a and an r2 of an output result, and a lower k place is used for accommodating an r1 of the input parameter b; t is a temporary calculation result of a polynomial, and the rest is intermediate variables. Calculation steps are as follows: calculating according to a formula that r2=MonPro(a,b,n)=a*b*2<-k> mod n, assigning zero for t and i, ensuring that r32=r[i], supposing the i to be 0 and starting external circulation, calculating according to formulae as follows: (c,z1)=t[i]+r[i]*r32, (c, z1)=z1*n0', x2=z1 and x1=r[i], supposing j to be 0 and starting the internal circulation, calculating according formulae as follows: (c, t[i+j])=t[i+j]+x1*r[i]+x2*n[i]+c and t[i+s]=c, adding 1 to the variable j, repeating the internal circulation when the j is smaller than the s, and quitting the internal circulation otherwise, adding 1 to the variable i, repeating the external circulation when the i is smaller than the s, and quitting the external circulation otherwise, judging whether the high k place of the t is larger than n, if yes, enabling that r2=r2-n and assigning the low k place of the t to the r2 otherwise, and outputting a modular multiplication result according to the r2. According to the method and the circuit thereof, the calculating speed of the algorithm can be increased, and the power consumption is reduced.