Ruth Davidson, Glasgow MSP, said even under plans to reform automatic early release there will be those convicted of serious crimes let out with years left on their sentence.
She said the Scottish Government's proposal is only for violent criminals serving ten years or more or sex offenders with more than four years, meaning many will still be back on the streets early.
Ms Davidson produced figures which show 97% of prisoners last year were given a sentence of less than four years.
She said the number, which totalled almost 15,000, included nine murderers, 603 convicted of serious assault or attempted murder, 183 convicted of sexual crimes such as rape and sexual assault and 628 convicted of knife crimes.
Ms Davidson said: "Criminals should serve the sentence they are handed down by the court.
"The fact this includes murderers and rapists, and those convicted of knife crimes, being allowed back on to our streets early is an insult to victims.
"Automatic early release also gives a false impression of offenders being given tough sentences and does nothing to help the role of prisons acting as a deterrent or a centre for rehabilitation."
Ms Davidson said if the proposal was in place now there would still be 80% released after serving only half their sentence.
She added: "These figures show the vast majority of criminals will still enjoy being let out of prison, no questions asked, after serving only half their sentence.
"The SNP promised in their 2007 and 2011 manifestos to end this controversial practise, and that pledge is as far away as ever from being delivered.
"The Scottish Conservatives have repeatedly called on the Scottish Government to scrap automatic early release but have been repeatedly voted down on this issue.
"However, we will continue to campaign to end this scandal to help make Scotland's streets safer."
stewart.paterson@ heraldandtimes.co ukco.uk