<p>Temperature effect in proto neutron star (PNS) PSR J0740+6620 is studied by using relativistic mean field theory and considering baryon octet. The nucleon coupling constant is chosen as GM1, and the temperature of the PNS is set as T = 7, 14, 21, 28, 35 MeV, respectively. Our results show that under the constraint of mass M = 2. 08 M_⨀ of the PNS PSR J0740+6620, the radius R increases with increasing temperature T. With respect to the same baryon density ρ, the field strength of mesons σ, ω, ρ and the chemical potential of neutrons μ_n decrease while the field strength of σ^* and ϕ increase with the increase of temperature T. The chemical potential of electron μ_e increases when the baryon density is less than ρ = 0.3 fm^-3, but decreases when the baryon density is greater than ρ = 0.3 fm^-3, companying with the increase of temperature T. Relative to the same baryon density ρ, the relative density of neutrons decreases while those of proton, Λ, Σ⁺, Σ⁰, Σ^–, Ξ⁰, and Ξ^– all increases with increasing temperature T. The central relative densities of neutrons decrease, while those of protons, hyperons Σ⁺, Σ⁰, Σ^–, Ξ⁰, and Ξ^– all increase with increasing temperature T. As for the relative density of hyperon Λ, it increases firstly, reaching a maximum value when the temperature is about T = 20 MeV, and then decreases with the increase of temperature T.</p>