  We use the universality of single soft behavior, together with the double
copy structure, to completely determine the tree amplitudes of non-linear sigma
model (NLSM). We first figure out the Adler's zero for $4$-point NLSM
amplitudes, by considering kinematics. Then, we assume the universality of the
Adler's zero, and use this requirement to construct general tree NLSM
amplitudes in the expanded formula, i.e., the formula of expanding NLSM
amplitudes to bi-adjoint scalar amplitudes. We also derive double soft factors
for tree NLSM amplitudes, based on the resulting expanded formula.
