Objective To investigate the effects of different salt ion concentration, polyethylene glycol (PEG) and force on structure of double DNAs braids. Methods Taking the 10 kb DNA as the research object, the effects of different concentrations of salt ions(Na+, K+, Mg2+), PEG and different forces on variation of relative extension of twisted double DNAs with rotation turns were investigated by flow chamber experiment of magnetic tweezers. Results The structure of double DNAs braids was sensitive to salt ion concentration but insensitive to PEG. With the increase of ion concentration, the extension of braids changed more gently with the rotation turns, and the electrostatic shielding saturation concentration of Mg 2+ was much lower than that of the monovalent cation. The effect of crowded environment on DNA was mainly the compression of contour length. The twisted structure of DNA was more stable under high force (above 4 pN), and fluctuated greatly under low force (lower than 2 pN). Conclusions The braiding structure and mechanical properties of DNA are affected by ion concentration in the solution and forces. The results may help to elucidate the mechanism of chromatin torsional torque affected by solution environment, and provide references for the function of topoisomerase under different solution conditions.