Court of Appeal judges sitting at Cardiff Crown Court said they would not grant leave for appeal.
One of Watkins' two female accomplices also had her appeal bid turned down.
Although 37-year-old Watkins did not challenge his 13 sex offence convictions, the disgraced rock star complained he was treated too harshly by sentencing judge Mr Justice Royce.
Watkins was handed a 29 year jail term as well as an additional six years on licence.
Among the offences he pleaded guilty to included the attempted rape of an 11-month old child as well as encouraging a fan to abuse her baby via webcam.
Originally, Watkins had pleaded his innocence to a string of offences - despite a damning video evidence.
He changed his plea at the last minute - averting a trial - but then complained he had not realised that his guilty plea had made him "look guilty".
Defending Watkins, QC Sally O'Neil said her client was not given enough discount for pleading guilty.
She said: "This case causes a very disturbing reaction for anyone who has anything to do with it. We accept these matters are very serious.
"But by pleading guilty my client averted the need for a most distressing and unpleasant trial for all those concerned, especially a jury.
"He did not have to plead guilty and if he had done so the consequences would have been shocking.
"I am not trying to minimise the gravity of these offences, just simply make the observation that a sense of proportion needs to be maintained.
"These offences, however bad, are not the worst."
But Lord Justice Pitchford said the sentences handed to Watkins and his female accomplice were appropriate.
Although a full judgment will be handed down in writing next week, Lord Pitchford said: "These offences against children were of shocking depravity...a very lengthy prison sentence was demanded."
The Court of Appeal also said Watkins did not demonstrate that his 29 year prison sentence was "not unduly manifestly excessive" and the sentencing judge had "reached the appropriate conclusion".
Watkins was not present at the hearing.