Protons are composed of three valence quarks. What is the relationship between the three valence quarks? In particle physics, it is difficult to find the mass relationship of three valencequarks in protons because the mass of quarks is not accurate. However, by analyzing the experimental results of seaquest carried out by Fermilab in 2021, it is found that the upper quark and the lower quark have an accurate mass ratio (mu/md=0.707). The exact ratio of the upper quark mass to the lower quark mass (mu/md=0.707) is another way to study the quark mass in protons. The discussion on Higgs mechanism and vacuum condensation of QCD can be avoided. The uncertainty of quark mass is transformed into the exact ratio of quarks in the same system to study. By analyzing the exact ratio of the mass of upper quark to lower quark in proton (mu/md=0.707), it is found that there is a relation of mu²+ mu²= md² in proton. The sum of squares of up quark mass in protons is equal to the square of down quark mass. The basic content of quark law in baryons: except protons, all 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. According to mu²+ mu²= md², it is deduced that there are three generations of protons in the universe. That explains why protons are stable. And analyze the reasons for the expansion of the universe.