This paper proposes a reasonable method for calculating the horizontal seismic action of precast concrete floors. This method can be used to ensure the integrity of these kinds of floors to transfer the horizontal force caused by earthquakes. Firstly, a typical 5-story frame structure model is built for further analysis. The acceleration values obtained by the elastoplastic time-history analysis are larger than those calculated using the equivalent lateral force (ELF) method or empirical amplification (EA) method which was proposed by ASCE 7-10. Therefore, both the ELF and EA methods underestimate the horizontal acceleration (i.e., horizontal seismic action) of the floors, which would lead to unsafe design in engineering practice. Secondly, the modal superposition (MS) method is proposed, in which the key parameters specified by ASCE7-16 were employed and modified to coordinate with the requirements of the seismic design codes in China. The MS method is employed for calculating the accelerations of the 5-story model in different seismic intensity areas. Lastly, the analysis results were compared with those obtained from the elastoplastic time-history analysis, which indicates that the amplification coefficients of the floor acceleration (k) have a similar distribution trend along with the building height. Moreover, the precast concrete floors designed using the MS method could meet the seismic performance objective “Keep elastic under moderate earthquake and unyielding under rare earthquake”.