The group counted nearly 50 community evictions last year alone, affecting almost 12,000 families.
A number of Asean countries have conflicts with China over maritime territories in the South China Sea.
Kasper-Ansermet was to have filled a vacancy created by the departure of German judge Siegfried Blunt - who resigned late last year.
Cambodia is pushing for a seat on the council, UN Security Council, for 2013 and 2014; voting will be held later this year.
HRW also detailed what it views as severe violations of human rights in nations including Cambodia, Indonesia, Malaysia and Vietnam.
The measures are designed to crack down on the sales of pirated U.S. products overseas.
A UN spokesman told VOA Khmer this week the government has an obligation to fill vacant posts at the court.
The letter was delivered to China, France, Russia, the UK and the United States.
Phan Imex was supposed to build 10 buildings to house evicted residents and built eight instead.
The UN appointed a reserve judge to the position, Laurent Kasper-Ansermet, but that appointment has not been confirmed.
Norodom Sihanouk, who is 90, has suffered from cancer, diabetes and hypertension and has made frequently, prolonged trips to China.
A US official said this week that Obama could participate in an Asean summit to be hosted in Cambodia in November.