The invention discloses a joint probability calculation method and system for the icing thickness of a power transmission line and wind speed. The method comprises the following steps that: on the basis of the simultaneous observation data of the icing thickness of the power transmission line and the wind speed, obtaining the annual maximum value and the 10-min average wind speed annual maximum value sample of icing thickness; according to extreme value I-type distribution, independently fitting the statistical parameters of the annual maximum value and the 10-min average wind speed annual maximum value sample of the icing thickness; calculating the correlation coefficients of the annual maximum value and the 10-min average wind speed annual maximum value sample of the icing thickness, and solving the control parameter of icing thickness-wind speed extreme value I-type joint probability distribution by the sample correlation coefficients so as to determine the icing thickness-wind speed extreme value I-type joint probability distribution; and according to an icing thickness-wind speed extreme value I-type joint probability distribution threshold value required by power transmission line engineering, giving icing area power transmission line design icing thickness and a simultaneous wind speed value, and providing a technical basis for the reasonable value of the icing area power transmission line design icing thickness and the simultaneous wind speed value.