The bombing occurred on Thursday in the town of Bannu, which has been a flashpoint for pro-Taliban fighters attacking security and civilian targets.
The attack came as the government announced that it believed Hakimullah Mehsud, the leader of Pakistani Taliban, was killed in a US missile attack last month.
No group has claimed responsibility for the Bannu bombing.
Police said the first bomb went off inside the police compound, while the second went off minutes later just outside the building.
The attack is the second against police in the last two days.
At least 19 people, including 11 policemen, were killed on Wednesday in a suicide car-bombing in Khyber, also in the northwest.
The blast occurred in the Wazirdand area of Khyber, a district that straddles a main supply line for Nato troops in Afghanistan.
There has been no claim of responsibility for that attack.
Fighters have carried out numerous attacks on security forces over the past several years to undermine the Pakistani public’s confidence in the already weak state.
The military has pursued the local Taliban in its main stronghold, the South Waziristan tribal region, for months.
But while it has managed to dismantle a good deal of its infrastructure, many of the fighters are believed to have fled to other areas in the tribal belt or just outside it.
Source: Agencies