Disclosed in the present invention is an application of Metrnl in diagnosis of atherosclerosis. A Metrnl-/-ApoE-/-double knockout atherosclerotic rat model and an EC-Metrnl-/-ApoE-/-atherosclerotic rat model are constructed. The experiment shows that the severity of atherosclerosis, the area of atherosclerotic plaque, and the degree of atherosclerosis and inflammatory infiltration in an aortic root of two Metrnl knockout rats are higher than a rat in a control group. The experiment shows that a vascular endothelial cell can secrete Metrnl, and the vascular endothelium and blood Metrin levels decrease during atherosclerosis, and the degree of decrease in Metrnl levels increases as the disease is exacerbated. The antithrombotic effect of a Metrnl recombinant protein does not have a species difference, and represents a dose-effect relationship on the inhibition effect of a clot contraction reaction. By comparing the function and characteristic of the Metrnl protein with a classical thrombolytic drug, it is found that the antithrombotic effect of Metrnl can reduce an adverse reaction of bleeding.