As the circular array frequently used in Mircrotremor Survey Method, this paper discusses the influence to measured results caused by different station number and arrangement on the circumference. With 10 stations arranged in circle for example, we estimate the SPAC coefficient from microtremor record, and compare the deviation of SPAC coefficient caused by different station number and arrangement on the circumference . The result is that when only one station on the circumference, SPAC coefficient can not be obtained accurately; when 3 stations are equally spaced, SPAC coefficient can be obtained accurately; when 3 stations are unequally spaced, SPAC coefficient can not be obtained accurately. The conclusion of paper is that 3 stations equally spaced on the circumference is the most suitable.