The invention relates to a low-alloy high-strength high-toughness steel plate and a manufacturing method thereof. The low-alloy high-strength high-toughness steel plate comprises the following components in percentage by mass: 0.08-0.25% of C, 0.10-1.00% of Si, 0.50-2.00% of Mn, less than 0.020% of P, less than 0.010% of S, 0.10-2.00% of Cr, 0-1.00% of Mo, 0-2.00% of Ni, 0.010-0.080% of Nb, at most 0.10% of V, at most 0.060% of Ti, 0.0005-0.0040% of B, 0.010-0.080% of Al, 0.010-0.080% of Ca, at most 0.0080% of N, at most 0.0080% of O, at most 0.0004% of H, and the balance of Fe and inevitable impurities. 0.20%<=(Cr/5+Mn/6+50B)<=0.55%, 0.02%<=(Mo/3+Ni/5+2Nb)<=0.45%, and 0.01%<=(Al+Ti)<=0.13%. The Brinell Hardness of the steel plate is 390-460 HB, the yield strength is 900-1100 MPa, the tensile strength is 1100-1400 MPa, the elongation percentage is 11-15%, the -40 DEG C Charpy V-notch longitudinal impact work is greater than or equal to 40J, and the plate thickness can reach 100mm above. Thus, the steel plate implements matching of high strength, high hardness and high toughness, and has favorable machining properties.