The ceremony is taking place on Sunday afternoon at St Mary Magdalene in Sandringham, Norfolk.
The Archbishop of Canterbury, Most Revd Justin Welby, will conduct the service as well as giving an address. He will be joined by Revd Canon Jonathan Riviere, the Rector of the Sandringham group of parishes.
Kensington Palace said William and Kate are pleased that the public will be able to attend. A spokesman said: "While the service inside the church will be private, The Duke and Duchess of Cambridge will be pleased to welcome well-wishers into the paddock outside the church.
"The Duke and Duchess are hugely grateful for the warm wishes they have received since Princess Charlotte's birth - many of them from local people in Norfolk - and are delighted the paddock can be opened on the day of the christening."
Police are advising those wishing to attend to arrive early.
Superintendent Kevin Clarke said: "With this being such a special occasion we are expecting the area to be extremely busy on Sunday and encourage the public to heed our advice so they can enjoy the event safely."
"People are asked to consider their arrival time carefully as they could be queuing for some time in warm temperatures with limited refreshments and facilities in the area."
The service takes place at 4.30pm and anyone bringing flowers will be asked to give them to representatives of East Anglia's Children's Hospices (EACH), where Kate is a patron, who will then take them to hospices in the region.
