Ajmal has made the squad despite being suspended from bowling in international cricket by the International Cricket Council since September this year.
"Saeed has been retained because there is still a month before teams have to submit their final 15 member squads to the ICC," a PCB official said.
"Saeed is in the process of working to correct his action and he has already shown a lot of improvement in his modified action. He will soon be sent for an official test to confirm whether he can resume bowling in international cricket," the official said.
They have retained left arm pacer Junaid Khan in the preliminary squad although he is still struggling to recover from a knee injury.
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Junaid, one of Pakistan's leading bowlers before he had to returned home midway through the Australian series from the UAE, has been advised a rigorous rehabilitation process to recover from the knee problem.
Chief selector Moin Khan said while announcing the preliminary squad that Junaid holds immense potential and promise to perform in any conditions.
Participating countries have to finalise their 15-member squads for the World Cup by January 7, 2015.
Pakistan preliminary squad: Mohammad Hafeez, Ahmed Shehzad, Nasir Jamshed, Sharjeel Khan, Sami Aslam, Misbah-ul-Haq, Younis Khan, Asad Shafiq, Azhar Ali, Sohaib Maqsood, Fawad Alam, Haris Sohail, Shoaib Malik, Umar Akmal, Mohammad Irfan, Wahab Riaz, Junaid Khan, Umar Gul, Ehsan Adil, Mohammad Talha, Saeed Ajmal, Zulfiqar Babar, Raza Hasan, Yasir Shah, Shahid Afridi, Anwar Ali, Bilawal Bhatti, Sohail Tanvir, Sarfraz Ahmad, Kamran Akmal.