Lead-free bismuth sodium titanate and related compounds are of great interest as promising candidates for piezoelectric applications. However, the full understanding of this family of materials is still a challenge partly because of their structural complexity and different behaviors with or without the application of an external electric field. Here, piezoresponse force microscopy is used to gain insight into the mesoscopic-scale domain structure of the morphotropic phase boundary (MPB) composition of (1-x)Bi0.5Na0.5TiO3-xBaTiO(3) solid solution at x = 0.06 (abbreviated as BNT-6BT). The evolution of the domains with the changes of the electric field and temperature has been thoroughly examined in conjunction with the crystal structure analysis and dielectric studies. It is found that ferroelectric domains with size of hundreds of nanometers are embedded in a relaxor state without visible domains on a mesoscopic scale, which are considered to contribute to the tetragonal and cubic phases in the material, respectively. Temperature-independent domain configuration is observed in the unpoled sample from room temperature to 200 degrees C. While, temperature-dependent domain configuration is observed in the poled sample. The homogenously poled state breaks into the mixed domain configuration containing polydomain structure and invisible state around the so-called depoling temperature. The structural changes on different length scales are also discussed. This work provides an in-depth understanding of the structural and domain changes under an electric field and the temperature-dependent domain evolution in both unpoled and poled states in the BNT-BT solid solution of the MPB composition.