Low-energy electron diffraction (LEED), Auger electron spectroscopy and ultraviolet photoelectron spectroscopy (UPS) were used to study the annealing effect of the silver ultrathin film on a Pt(111) surface. The intensities of the specular beam of LEED versus temperature have abnormal change between 550 and 650 K in a submonolayer range. The irreversibility of this I-T curve gives evidence of a structure change of the surface. The intensity of the specular beam of LEED varying with annealing time plus the Auger signals of Ag and Pt function of temperatures show that a disordered two-dimensional (2D) Ag-Pt alloy is formed in the submonolayer range at a temperature between 600 and 700 K. UP spectra in the growth process show that the d band and the Fermi edge have some interesting changes. On the other hand, UPS in the annealing process also strongly suggests that Ag and Pt atoms mix on the top layer to form a 2D surface alloy at a temperature between 600 and 700 K and a coverage in the submonolayer range.