With the fast development of high voltage DC (HVDC) cable, cable insulation under DC conditions has got more attention. In this paper, tests were conducted to study the electrical tree initiation in silicone rubber (SIR) under DC and polarity reversal voltages. It is found that electrical tree initiation has significant polarity effects under both DC and polarity reversal voltages. There are only single-branch-like trees and branch-like trees under DC and polarity reversal voltages. As for pre-stressing effects under polarity reversal, the pre-stressing voltage has positive effects to electrical tree initiation, while the pre-stressing time has little influence. Space charge distribution of SIR under high electric field was studied with flat plate pulsed electro-acoustic (PEA) system, and their characteristics were discussed to explain this phenomenon. Moreover, different treeing breakdown phenomenon is found under polarity reversal voltage which differs from that under DC voltage. The existence of fast charges and slow charges gives reasonable explanation to it. Special attention should be paid to the transient situation like polarity reversal which would result in irreversible effects more easily, and affect true length of life for HVDC cables. (C) 2017 Elsevier B.V. All rights reserved.