The existence of ultrahigh voltage direct-current (UHVDC) transmission lines impacts geoelectric field observation (GFO), which further disturbs earthquake observation and prediction. We proposed a mathematic method based on a 3-dimensional layered soil model to calculate the interference and conducted GFO experiments at observatories and field observation sites during the live debugging of a $\pm $800 kV UHVDC transmission line to verify the method. The results obtained by using the proposed method are in good accordance with the experimental results and the maximum error is no more than 15%. Moreover, the experiment proves that grounding current is a decisive factor in the interference from a transmission line to GFO.