Abstract: The present invention provides a selection method of array length of observation system, comprising: step S101: obtaining a seismic response equation for any point on ground based on wavefield propagation theory; step S102: determining at least one selection criterion of an optimum array length considering different factors according to the seismic response equation; step S103: obtaining the array lengths considering the different factors respectively according to the at least one selection criterion of the optimum array length; and step S104: integrating the array lengths considering the different factors and determining that the optimum array length is ?{square root over (2)} times the depth of the destination layer. Compared with conventional array length calculation method the demonstration method of array length as proposed in the present invention has the advantages of higher accuracy and applicability for destination layers.