In this paper, we reestablish the elementary Darboux transformation for Sasa-Satsuma equation with the aid of loop group method. Furthermore, the generalized Darboux transformation is given with the limit technique. As direct applications, we give the single solitonic solutions for the focusing and defocusing case. The general high order solution formulas with the determinant form are obtained through generalized DT and the formal series method. 1975 1976 LIMING LING then equation (2) could be rewritten asSSE (4) is one of a limited number of integrable models and has been a field of active research for the past two decades. Thanks to the integrability, the sophisticated soliton construct underlying this wave equation can, therefore, be achieved using an array of mathematical tools such as inverse scattering transform [22,14,17,33], Darboux transformation (DT) [27,30,1,4,32,25], Hirota bilinear method [6,5,26], and others. Although the DT for the SSE was given in the previous study, the reduction of DT is not complete. In this work, we reestablish the DT through the loop group method [29]. With this way, we can obtain the DT completely. On the other hand, since the spectral problem for SSE (4) is 3 × 3 and possesses deep reduction, the high order DT can not be obtained directly. Based on the elementary DT [21,7] and analysis of spectral parameters, one can obtain the generalized DT [2,9,10,19]. As direct applications, the single solitonic solutions, multi-solitonic solution and the high order solitonic solutions are given through this method.In the previous literature, most of works concentrate on the soliton solution for SSE on the zero background [27,28]. Recently, there are some works focus on the solutions of SSE with the non-vanishing background (NVBC). For instance: the breather solution [30, 1], W-shape soliton [32], rational W-shape soliton [34] and the twist rogue wave solution [4] are obtained for the focusing SSE in the recent literature. In this work, we construct the solitonic solution for both the focusing and defocusing SSE on the NVBC systematically. For the focusing case, when the spectral parameter is located in the image axis, we can obtain the soliton solution and resonant soliton solution, periodical solution, half periodical solution, rational W-shape soliton, resonant rational W-shape soliton, and their high order ones; when the spectral parameter is not located in the image and real axis, we can obtain the breather solution, resonant breather solution, rogue wave solution, composite rogue wave solution and their high order ones. For the defocusing case, when the spectral parameter is located on the segment of real axis, we can obtain the dark soliton, W-shape dark soliton; when the spectral parameter is not located on the real and image axis, we can obtain the breather solution and its high order ones.The structure of this paper is organized in the following: In section 2, we give the DT and generalized DT for SSE through the loop group method. The reduction for DT was analyzed by spectral...