An initial military inquiry into the death of 18 Palestinians, mostly women and children, on Wednesday attributed the shelling to a "technical failure of the artillery radar system".
But HRW said that the army's investigation "failed to address the key questions of whether the attack was a violation of international law and who should be held accountable for the lethal fire".
"The Israeli government should immediately conduct a comprehensive independent investigation to establish these issues," HRW said.
Sarah Leah Whitson, the director of the Middle East and North Africa Division of HRW, said: "The IDF's (Israeli army's) internal probe suggests that the Beit Hanoun tragedy can be chalked up to an errant volley of shells.
"But a comprehensive investigation should start with questioning whether Israel had any business firing artillery shells into this civilian area to begin with."
HRW said the inquiry should examine the Israeli policy which led to the firing of "some 15,000 artillery shells" into Gaza since September 2005, killing more than 49 Palestinians and seriously wounding dozens more.
Whitson said: "Israeli forces launched the artillery attack on Beit Hanoun at a time when their commanders knew, or should have known, that the risk of civilian deaths far outweighed any definite military advantage."
Amir Peretz, Israel's defence minister, ordered stricter artillery firing rules for the Gaza Strip on Thursday.