A temperature field theoretical model of classical telescope primary mirror is presented. Analytical solution is gotten by separation of variables and Green′s function method. It is easy to get the temperature difference between primary mirror work surface and ambient air with the analytical solution of the temperature field. A quantitative analysis of the primary mirror seeing is addressed. The relationship between primary mirror seeing and the size of primary mirror is studied in two different materials (fused silica and BK7) and two different kinds of mirror (traditional mirror and thin mirror) in two definite conditions with analytic solution. For 4 m traditional mirror and thin mirror, the relationship between primary mirror seeing and descending slope of the ambient air temperature is also presented. The quantitative analysis is helpful for estimating primary mirror seeing. The analytical solution of primary mirror is also valuable in designing telescope primary mirror for calculating thermal stress and thermal strain.