How to extract the mass of up quark and down quark in proton? In this paper, a new extraction method is found. Although the exact mass of the up quark and the down quark in the proton cannot be obtained, the accurate ratio of the mass of the up quark and the down quark can be extracted, and the mass relationship among the three valence quarks can be found. Based on the experimental results of Seaquest carried out by Fermilab in 2021, through analysis, the accurate ratio of up quark to down quark mass (mu/md=0.707) is obtained, and the mass triangle is established. It is deduced that the sum of squares of up quark mass in protons is equal to the square of down quark mass. The mathematical expression is mu²+ mu²= md². According to the observation of the decay law of other baryons, the quark law in baryons is obtained. The basic content of quark law: except protons, the heavy quarks in other baryons will decay into up quark or down quark, and the sum of squares of up quark mass should be equal to the square of down quark mass.