Abstract: For some flexible, slender structures such as chimneys, generally three modes are needed in calculating the horizontal components of earthquake force based on the theory of response spectrum, a process more or less tedious. Consequently, some simplified calculation methods are suggested in our design code of earthquake-resistant structures in order to save engineers'time of design, although it is claimed that mode shape analysis should be followed as a general method in calculating earthquake force. This paper contains two aspects in this regard and as follows:(1) Using mode shape analysis method as a guide, this paper criticizes at first the remarkable error introduced by the calculation method originally recommended by the California lateral force calculation code of 1952 and followed later by many countries in their corresponding codes including that adopted by our country in 1974 with some modifications saying that the base shear should be calculated in the first step and then distributed it along the height of the structure.(2) Two simplified calculation methods are recommended here which are very easy to be handled and give errors much less than the two methods mentioned above.