Nitrogen removal via autotrophic denitrification, an anaerobic ammonium oxidation (Anammox) process, requires an appropriate NO2-N/NH4-N ratio to provide nitrite as an intermediate. In this study, a laboratory-scale Hybrid Sequencing Batch Reactor (HSBR) was implemented for treating high-ammonium raw leachate to yield an appropriate NO2-N/NH4-N mixture as a pretreatment for subsequent Anammox. The optimal operating conditions were examined through the long-term operation of the HSBR. The experimental results showed that the prerequisite ratio of NO2-N/NH4-N was found with an initial ammonium concentration of 1200 mg/L, dissolved oxygen (DO) of 0.5-1.0 mg/L, sludge retention time (SRT) of 3 days and temperature of 31 degrees C, which is essential for the subsequent Anammox process. Moreover, the inhibition of free ammonia (FA) and free nitrous acid (FNA) were also examined under a constant pH condition, and it was found that AOB (ammonium oxidation bacteria) had a great ability to adapt to a broad FA and FNA concentration, whereas NOB (nitrite-oxidizing bacteria) were inhibited by either FA or FNA concentration to a certain extent. It appears that partial nitritation could be implemented by facilitating FA and/or suppressing FNA concentration.