This paper analyzes and calculates the influence of the pendulum angle，the rotation of

the ball，the mass of the suspension line，the air buoyancy and the air resistance on the period of

the pendulum one by one. Tak- ing a 1 m length and 5° swing angle small amplitude pendulum as an

example，the relative errors of the five factors are

+0.45‰，+0.02‰，-0.45‰，+0.07‰，-0.14‰，respectively，and the total is only -0.05‰，which means that the

five error factors almost cancel each other，and the systematic error of the small

pendulum experiment is very small in theory. This paper gives a more detailed

reasoning，calculation and analysis process，especially the reasoning process of air resistance

moment，as well as charts and tables，for providing a more complete and accu- rate

understanding. Finally，some necessary explanations are given in view of some possible deficiencies

in people's understanding of air resistance.