On bounded and simply connected planar analytic domain Ω , by 2π periodic regular parametric representation of boundary curve ∂ Ω , complete convergence and error analysis are done in L 2 setting for least squares, dual least squares, Bubnov-Galerkin methods with trigonometric polynomials into Symm's integral equation of the first kind KΨ = g when g ∈ H r (0, 2π), r ≥ 1.In this paper, we focus on the numerical behavior of (LS), (DLS), (BG) when g ∈ H r (0, 2π), 0 ≤ r < 1. Weakening the boundary ∂ Ω from analytic to C 3 class, it is proven that the (LS), (DLS), (BG) with trigonometric basis will uniformly diverge to infinity at first order. The divergence effect and optimality of first order rate are confirmed in an example. In particular, we show that the strong ellipticity estimate and Gärding inequality are also powerful in divergence analysis of Galerkin method on ill-posed integral equations.Grants or other notes about the article that should go on the front page should be placed here.