Optical nonlinear absorption properties of the solution in chloroform, the doped films in PMMA, the spin-coating and casting films of soluble poly(3-hexyl-2,5-thienylenediethynylene) (PHTE) were studied on a Q-switched Nd:YAG laser at 532 nm with 10 ns pulse width. It has been found that the optical nonlinear absorption properties of PHTE were affected greatly by the self-assembly effect caused by the D-A interaction strengthened pi-stacking interactions among the PHTE molecules. By tuning the interaction strength among the PHTE molecules, it can show the optical bleaching effect and the optical limiting effect, respectively. These might be attributed to the self-assembly among the PHTE molecules having reduced the lifetime of the first singlet state S-1. The special properties of PHTE will afford it important potential application in nonlinear optics.