The invention discloses a pole-side conductor sag observation method based on a total station instrument, which is suitable for performing conductor sag observation at the external side of pole span of an electric transmission line with the total station instrument. The total station instrument is disposed at the external side of the pole span to respectively detect the altitude differences of a conductor wire at mount points on a first pole tower and a second pole tower by means of a prism-free function; by means of a planar triangle formed by the observation point and projections of the mount points on the two pole towers, first planar distance and second planar distance are measured; according to geometric principles, projective triangle interior angles and pole span length are calculated and further the position of midpoint of the pole span is determined; the length from the observation point to the midpoint of the pole span is calculated and is multiplied by the tangent value of elevation angle of the total station instrument to calculate the altitude difference of the conductor wire at the midpoint of the pole span; the average value of the first and the second altitude differences minus the altitude difference of the conductor wire at the midpoint of the pole span is finally calculated to obtain the conductor sag. Compared with a conventional conductor sag measurement method, the method allows optional site selection, has high precision and is simple in operations, and can be used in the fields of ultrahigh-voltage transmission line construction, overhead transmission line engineering construction, and measurement and control technologies and instruments and the like.