In this talk, I will present two of our recent research results: (i) DNA deformations induced by salt and temperature; (ii) DNA knotting. For the first topic, our collaborator (Prof Xinghua ZHANG in Wuhan University) used single-molecule magnetic-tweezers to measure DNA twist changes induced by salt or temperature change. We revealed the mechanisms underlying these DNA deformations using molecular dynamics simulations [PNAS (2023), Science Advances (2022), PRL (2022)]. For the second topic, we developed a statistical mechanical model to describe DNA knots, which can explain the probabilities, sizes, and shapes of DNA knots under various conditions.