FDPM and FOOJA, which belong to the low complexity class, are the most efficient algorithms for principal or minor subspace tracking. Reducing computational cost is crucial for guaranteeing the real time implementation. Under lower computational complexity background, it was found that there exist two other simplified methods (FDPM＿1st＿col and FOOJA＿1st＿col) through analysis of FDPM and FOOJA. Under the finite word length condition, the stability and numerical robustness were analyzed and compared with the former two algorithms. Simulation results verified our conclusion.