Yeung Ki, 41, admits killing Zhou Hui, 32, in December 2012 but claims her actions were provoked. She told police Zhou beat her and her four-year-old daughter after she refused to give him money to buy a car.
She also said he forced her to have sex with him, the South China Morning Post reported Tuesday.
Prosecutor Nicholas Adams said in court Yeung fed Zhou soup laced with sleeping pills before striking him repeatedly with a hammer, adding, "She also cut off his penis and flushed it down the toilet."
Zhou, a married resident of the Chinese mainland, had borrowed money from Yeung and made her pregnant before returning home to the city of Shenzhen. Claiming her missed her, Zhou returned to Hong Kong to visit her several times, the newspaper said.