Aiming at the problem of electric field distortion and abnormal temperature rise caused by insulator degradation in DC transmission system, a distributed conductance matrix was constructed based on the multi-conductor system theory, and the distributed conductance characteristics and electric field distribution of U550BP/240T porcelain insulator string under DC voltage were studied by electric field simulation. The heating mechanism of insulator string was revealed by thermal-electric coupling model and DC live test. The results show that the distributed conductance between normal insulators is basically equal, about 15×10^-10 S/m, while the conductance between the fittings of zero-value insulators increases sharply to about 108×10^-10 S/m. The distributed conductance between fittings and conductors, as well as between fittings and ground, decreases with the increase of spatial distance. The voltage along the insulator string shows a U-shaped distribution. For the zero-value insulator, the partial voltage drops to 1.4%-1.6% of the total voltage, and the peak value of axial field strength attenuates to 5.5%-7.1% of the normal value, resulting in a significant rise in the field strength and partial voltage of the adjacent insulator. Through the 120 kV DC live test, it is verified that the heating curve along the string is similar to the U-shaped distribution curve of voltage, which verifies that the voltage plays a leading role in the heating of insulator. The iron caps of zero-value insulators exhibit uniform heat generation, while those of normal insulators show a significant temperature rise in the middle and lower parts. The correlation coefficient between the heat generation characteristic curves of the insulator string obtained from simulation and experiment is greater than 0.95%, with a maximum error of 12.38%, which verifies the decisive role of distributed conductance in the electric field distribution and heating of insulator, and can provide theoretical basis for the state monitoring of insulators in DC system.