In this paper, two new series of main-chain and side-chain second-order nonlinear optical (NLO) polyurethanes have been successfully prepared, in which isolation groups with different sizes were introduced to adjust the NLO property of the resultant polymers, according to the concept of "suitable isolation groups". The second harmonic generation (SHG) experiments demonstrated that all the polymers exhibited large second harmonic generation (SHG) coefficients of d(33) values (up to 105.6 pm/V) with excellent thermal stability and film-forming ability, indicating that the introduction of isolation groups could alleviate the "nonlinearity-stability trade off" efficiently. In addition, for both main-chain and side-chain polymers, BOP acted as the suitable isolation group. It was also found that the positive influence of the linked suitable isolation spacer was more obvious in the main-chain polymers, in comparison with those in the side-chain polymers.