The overall convergence performance of the unified iteration methods is better than that of the alternative iterationmethods for load flow calculation of AC/DC power systems. But for some ill-conditioned or unsolvable cases， the unifiediteration methods may be divergent， and could not give useful information. The widely used optimal multiplier in power flowcalculation for AC power systems is expanded to the hybrid AC/DC power systems. According to features of DC system steadyequations， auxiliary variables are introduced to transform them to equations with quadratic or linear forms. The implementationapproaches of Newton load flow methods with optimal multiplier in rectangular and polar coordinates are given. Test resultsshow the effectiveness of the proposed approaches， and the results obtained in rectangular and polar coordinates are comparedand analyzed.