This study relates to the architectural planning principle of Gyeonghoeru pavilion, one of the major buildings in Gyeongbokgung palace. The study is concentrated on The Book of Gyeonghoeru Pavilion written by Jeong Haksun in 1865 during the reconstruction of Gyeonghoeru pavilion. The architectural planning principle of Gyeonghoeru pavilion disclosed in The Book of Gyeonghoeru Pavilion appears to be 'the method of expanding with six by six system'. 'The method of expanding with six by six system' means a way of continuously extending the number related to 6 by addition, subtraction, multiplication and division, and reflecting to architecture, having number 6 as the basic number. Number 6 means a large amount of water in East Asian philosophy, Applying 'the method of expanding with six by six system' to the architecture of Gyeonghoeru includes an intention to prevent fire in a way of a type of incantation because Korean traditional wooden buildings are we to fire. Since Gyeonghoeru is surrounded by a pond and was constructed based on the number 6 having a strength of water, it was believed that it could be safe from fire. This study compares the contents disclosed in The Book of Gyeonghoeru Pavilion with the construction of Gyeonghoeru to find out how the number 6 was applied to the overall construction of Gyeonghoeru. From the fact that the total number of km of Gyeonghoeru is 36 ($6{\times}6$), the number of pillars is 42 ($6{\times}7$), and the number of windows and doors is multiples of 6, it has been found out that the number 6 is deeply related to the overall construction of Gyeonghoeru. In addition to the fact that the construction of Gyeonghoeru can be explained by 'the method of expanding with six by six system', The Book of Gyeonghoeru Pavilion also discloses parts where Diagram of Hotu, Later Heaven Arrangement and Arrangement of 64 Hexagrams are applied. Therefrom, it has been found out that Gyeonghoeru pavilion was constructed by applying the principles of East Asian philosophy based on The Book of Changes.