The present invention provides an antibody specifically binding to PD 1 with high affinity or a functional fragment thereof. Further provided are a nucleic acid molecule encoding the antibody of the present invention or the functional fragment thereof an expression vector and a host cell for expressing the antibody of the present invention or the functional fragment thereof and a method of producing the antibody of the present invention or the functional fragment thereof. Further provided in the present invention are an immunoconjugate and a pharmaceutical composition containing the antibody of the present invention or the functional fragment thereof as well as a method for treating a plurality of diseases (comprising cancers infectious diseases and inflammatory diseases) by using the antibody of the present invention or the functional fragment thereof.
