In a DC transmission system based on multi-source self-adaptive line commutated converters (SLCC), commutation failure can be suppressed by configuring STATCOM to compensate reactive power under AC faults. However, under severe AC faults, commutation failure may still occur even when the STATCOM reaches its maximum compensation capacity. To address this issue, a commutation failure suppression method based on STATCOM commutation current compensation is proposed. First, the topology and control strategy of the SLCC-based DC transmission system are analyzed. Second, by examining the SLCC commutation process, it is found that the LCC commutation current is jointly provided by the AC grid and the STATCOM. Based on this, it is proposed to compensate the LCC commutation current by controlling the STATCOM output current to accelerate the commutation process and reduce the commutation overlap angle, thereby suppressing commutation failure. Compared with reactive power compensation based methods, the proposed method achieves better commutation failure suppression under the same STATCOM capacity. Finally, a SLCC DC transmission system model is built on the PSCAD simulation platform. Simulation results show that the proposed method can effectively enhance the ability of SLCC to withstand commutation failures.